While on vacation my taxi took a longer route, possibly to scam me out of money. How can I deal with this?

This is actually based on my mother's experience, and not my own.

Yes, this is a common scam. Despite some answers wanting to be generous and say it's possibly accidental, much of the time it is not.

If I remember correctly, she told me the way she got around it was by telling the drivers exactly what route she wanted them to take. Even then they would try to add a few streets to it to up the charge. Fortunately for her, once she learned to speak Türkçe more fluently, the drivers started looking at her as a local and not as a tourist which resulted in them being more fair to her as well.

In fact, you'll find a lot of places in Istanbul have a "Local's" and a "Tourist's" price. You can easily save yourself quite a few Lira even on a cup of tea just by learning to be conversational in the Turkish language and asking if the price they are giving you is really the price they would charge someone from the area. Nowadays, my mother is pretty much just accepted as Turkish whenever she goes over there. These scams really just rely on your ability to call them out. If they know you know what they are doing, they won't be so quick to try and scam you.

The problem is, not everybody can learn a new language, especially if they only plan on going to the country once or twice. If you're already there, this will not help you aside from "Tell them your preferred route." It's honestly the best and only way you can do so without basically scaring them into thinking you're a local.

Also, never hop into a gypsy cab in Istanbul. EVER. Only use an official taxi or a trusted ride-sharing service like Uber or Lyft. (If you do take Uber or Lyft, double and triple check that the driver and vehicle you get match what the app says or refuse the ride, take a photo of the vehicle, and tell the app to send you somebody else, requesting a refund for any cancellation fees since it's not your fault the information didn't match.) Do not EVER take a ride in a taxi that 1) isn't yellow, 2) lacks the "taksi" sign on the roof, 3) doesn't display its associated company on itself, and/or 4) lacks a functioning meter. If even ONE of those factors isn't accounted for, get out of the vehicle and flag down another cab. At best, you will get ripped off by that cab. At worst... you've heard the stories of the dangers of hitchhiking. Be smart, stay safe.

If you aren't using a ride-sharing or taxi app, as suggested by jpatokal, use a different app: a mapping app. Enter your destination (make sure it's correct), show the map to the driver and ensure he/she has a shared understanding of where you want to go, and keep an eye on the route. If the driver has a GPS, ask that they use it. Keep in mind that there may be legitimate reasons to use out-of-the way routes, some of which may not be obvious to a visitor, but if you're straight up going to the wrong part of town, ask that the problem be solved immediately. If you're not comfortable with the situation, and the area you area you are in is not immediately unsafe, stop the ride, pay for the services you were given, and get a new ride from another driver.

Since you're staying at a hotel, this may also be something the hotel staff can help with when you do arrive. They may not want to get involved, and I can certainly understand that, but doormen at nice hotels in countries known for taxi scams will sometimes make it clear to drivers that their guests need to be treated well and help intervene in disputes (of course, in other places, the doormen may conspire with illegal taxis for tips).

Not a complete answer, I admit.

The advice I was given when visiting Istanbul is that taxi drivers often appear not to be able to read - and to be prepared for that. The advice was to give them both a printout of the address and also a simplified map, showing them which roads I expect them to take to my hotel.

That the driver couldn't read could be a charitable explanation of how your driver took you to the wrong place despite you showing him the card. According to the comment from @Ege Bayrak, this is the essence of a popular scam.

This has happened to me many times in Cuba where it’s accepted practice to overcharge tourists, especially if you don’t use a licensed taxi (which, of course, doesn’t remove the risk that the driver will take the scenic rather than direct route). None of the Internet-based suggested answers really work in countries like Cuba where online connection is difficult/unreliable/expensive.

I’ve learnt to deal with this situation by always asking for the price upfront, and before you get into the taxi. It also helps to know how to ask ‘how much to XYZ’ in the local language and to find out beforehand from a reliable source eg internet search before you travel, or from the hotel where you’re staying, how much the taxi should cost, so that you know if you’re being over-charged and can decide whether you want to pay it or not before you take the ride. Even licensed taxis will inflate the price if they think they can get away with it eg I was quoted 120cuc for an airport taxi on my last trip but I know the norm is 90-100, which the driver agreed to when he realised I’m not quite as daft as I might look.

It often helps to ask the price of your ride in advance, before getting into the car. Walk away if the price is too high or the driver can't provide one. If you know the right price, don't hesitate to name it. As a safeguard against misunderstanding, give them the street address and ask if they know it. If they don't reply with the hotel name, give them a printout of your booking, mentioning the hotel's name and address. Record the conversation if you're paranoid.

I managed to get quite close to the "Local's price" with this strategy in many places (though not specifically Turkey) despite I didn't speak the language and clearly was a tourist. And in case the driver took me to the wrong place, or took a detour to increase the price, I'd have his words to use as an argument.

First off, there is not "polite" way to deal with that. If it is a scammer, they won't refund you if you ask nicely. If it was a misunderstanding, they will still be offended.

Your options are basically to pay up, or make a scene. At which point other people (including police) may get involved who may or may not speak your language, and who may ore may not be on your side.

As far as Instanbul goes, Wikivoyage has some good instructions on taxis. The gist is: Know the price, be aware where you are going, have them use the meter.

Their recommendation is to ask for an estimate beforehand and then ask them to put on the meter. If you agree to a "fixed" price it'll often be higher than the metered fare. And always use legitimate taxis - see Sora Tamashii's answer.

There are also apps that allow you to download a map for offline use, if you don't have a data connection, like HERE WeGo.

My limited experience is that Taxi drivers in Istanbul often use Google Maps and were willing to show the destination and route on their phone if you don't have one.

This is pretty typical for Near/Far East/Asia, and it gets more pronounced the farther east, and south, you go. Also, the poorer the place is overall, the more pronounced.

Your question "How do you deal in a polite way" already shows the main issue. That is, pardon me for being blunt, a facepalm statement.

You only have two options (well, three actually):

1. You just pay.
2. You are prepared. And no, politeness is not part of your preparations.
3. You stay home.

Being prepared means you have an approximate idea of where you are and what -- approximately -- it will cost you. Ask at the hotel if unsure, they'll know. With cell phones, GPS, and Google Maps, this is nowadays a trifle, anyway. But it was easy and doable 20 years ago with a map on paper, too -- no problem. It's not like you drop by parachute at a haphazard random location in an unknown city. You have a rough idea about where you're going ahead of time.

You have the adequate amount of money ready, and not in your wallet. If necessary, get a couple of small bills in your hotel for that very purpose. You must expect not to get change (it's a fair assumption that any bill that ever leaves your hand will never be seen again), so large bills necessarily mean you will pay more.

Before going, make sure the driver says that they have understood where to go, and that a price which you are willing to pay (probably 50% more than you should pay anyway) is agreed on. Show the driver that money and be explicit that this, and nothing else, is what he will get, no more. If the driver doesn't want to agree, walk away.

Do not, ever, ask or leave any doubt about what you're going to do or pay (you may ask for the price, but if that is more than you are willing to pay, which is pretty certain, it must be 100% clear what you will be paying, and that you will not make a compromise). Do not, ever, show that you are uncertain.

Do not get your wallet out, do not ever let them look inside or even see it. You do not even have a wallet. You do not have any other money. This is the only money they will ever see, or get. Maybe, that is. If they do as you say.

Inevitably, they will start to haggle. Do not engage in this. It is "Yes or Yes", not anything different. Inevitably, they will haggle again after arriving at the destination. Do not engage in this. Give them the money you agreed on, turn around and go.

You are not polite, do not even attempt to be. You need to realize that you are not at home, and you are not among friends. The social rules of interaction are not what you are used to.

If you are trying to be polite, you are weak, and there's only two kinds of people in the world: those whom you exploit, and those that you respect. There's those who (figuratively) slap you in the face, they're the ones you respect, you are grateful to them for slapping your face. And there's those who you steal from. Because, well, they're weak. They're victims.

This is harsh, and it may not be agreeable with your normal attitude (it took me a long time to learn this, too), but it is reality. If you cannot adapt to this attitude, then you either shouldn't travel, or you should just accept that you will be ripped off, and pay. Because either way, you're going to pay. Because you are a victim, you just haven't understood that yet.

Note that the same thing can happen to you in Europe as well (doesn't have to be Asia, only just it's much more common there). Happened to me, much to my surprise, in Bordeaux 6 or 7 years ago.

There is no polite way to do this. As a Turkish citizen I also not want to use taxi in Istanbul because you need to argue with them to not get scammed. Just say I'm calling cops and determinedly call 155, then you can solve your problem. If he doesn't step back until the police came, insist on what he was doing because he will probably lie about the situation. You may go to your way after police officers charge him with an administrative fine.