There is one way to know for absolute sure. As you are checking in with a human, who is preparing your bag tag, you ask "do I need to interact with my bag at [transit airport] or will I just see it again at [final destination]?" They will give you a definitive answer.

However, you may not plan to check in with a human (many kiosks print bag tags and then you put your bag on a belt without talking to anyone) or you may want to know in advance. This is not a generally answerable question, but:

• if you bought two separate plane tickets, one from A to B and one from B to C, then you will definitely have to help your luggage make the connection.

• if your transit is in a different country than both your departure and final destination, you can find out from the airport if you need to help your luggage make the transit. Find the airport's web site and search for Connections.

• if your transit is your first point of entry into a country , and your connection is a domestic flight, you will definitely have to claim your bags and clear customs with them

• if your transit is your first point of entry into Schengen, and your connection is an inside-Schengen flight, you will definitely have to clear immigration, and you may need to claim your bags and clear customs with them. Check with the airport (as in the three-country case above) to be sure.

• if you want access to your bags in the transit airport you can request it (this will probably involve checking in with a human)

• most other situations will not involve you having to interact with your bags. To be sure, ask the airline that operates your first flight, using whatever contact mechanisms you can find on their web site.

So if you're in the last camp, there's no clear and obvious reason why you can't expect your bags to make the connection without your help, but if you're worried and want reassurance before you fly, ask that first airline. They will know if they intend to pass the bags along for the next leg or deliver them to you.

Note: some combinations of airlines and airports may be able to do things that other combinations cannot, such as transferring your luggage for you even on separate tickets, or letting you clear customs without claiming your bag (countries will not let you mix with domestic passengers before you have cleared customs, but they can trust your description of what's in your bag and see you again after you've claimed it.) You are more likely to see this from airports and airlines trying to encourage using them for transits: Turkish is a good example of this. If in doubt, ask your airline and/or the transit airport.

Before beginning, I would like to kudo travel-dealz.eu, whom I am not affiliated with, for a good and comprehensive article that I cited in this answer.

Let's start with the basics.

What is check-through and how does it differ from regular check-in?

Suppose you fly from X to Y via Z. Z is your flight connection airport. X, Y and Z can be any domestic or international airports, not cities. If your itinerary includes changing airport within the same city (e.g. London Heathrow to Stansted, New York JFK to LaGuardia), this will never apply

If you check-in your bags at every airport, you must 1) deliver your pieces of checked bag at X, 2) collect those bag(s) at Z, 3) go and line up at Z's bag drop counters, hand over the bags, 4) collect the bags at Y which is your final destination

If you can check-through, the ariport handling operator at Z will collect the bag from the inbound aircraft and deliver it to the next outbound aircraft of your trip, so only steps 1) and 4) from the above are required. You can be in the gate area at Z and wait for your next flight, saving time and physical effort in handling the bags.

I have the following itinerary (...), can I check bags through?

The answer is that there is no final and generic answer that can be given on a travel board. Surely, users can post by their own experience, but no answer can fully cover all cases, and you really need to find someone who traveled the same itinerary before you on your same conditions (see further questions) to get a reliable answer. An answer that can change anytime because of airline policies

Check-through with different tickets?

That is a 99% full NO. If you buy separate ticket, you normally have no way to check the bag through. Those are separate transportation contracts. The linked article claims that you have a lot more chances to get your bags checked through, but that's just not the normal rule.

In fact, it is claimed that in case the airlines have agreements, including code-share or being members of the same alliance, you have more chances to convince a check in operator by asking nice. But that is a claim, which can be true a lot of times, not consistently true.

Check-through with low-cost airlines?

Low cost airlines are particularly known for their cut-costs policy. Your chances of a check-through with Ryanair or WizzAir (for example) are close to zero. Handling bags is a cost

If one of your flights is booked with a low-cost carrier, your chance of getting your luggage check through are practically zero. Most of the no-frills carriers do not sign agreements with other airlines. So even if the agent on the ground would like to help you – which per se is already rather unlikely – they can’t.

Check-through and customs?

Your ability to check-through is affected, in a negative way, by certain customs policies on international travels. Nobody can likely can tell in advance the whole list of points, but once you have to clear immigration in certain countries, you must do with all your pieces of bags. That is specifically the case of US, Canada and China (see here). Probably a lot more cases.

Maybe I can tell that if you fly to the Schengen area, with Y and Z being any Schengen destination, you clear immigration at Z but then clear customs at Y (I may be partially wrong, but we could discuss Switzerland, not-EU-but-yes-Schengen, as a great example all night...)

So if you find that your itinerary is comprised of certain destinations which require customs for transit travelers, you found your NO

>>> Who can tell me for sure? <<<

Not a travel board can tell you for sure whether you can get your bag checked through. If you bought a single ticket there are two only reliable ways

1. Call the airline, introduce your PNR to them. They will know for sure

2. The final way is to check your bag tag and ask the bag-drop attendant.

Only they know, at the time you travel. This because, for any special circumstance, your check-through might be downgraded last minute to check-in. And you came to the airport all happy that your bag will fly X->Z->A->B->C->Y and forget that you must retrieve it at all or almost all stops, losing your bag than asking a simple educated question.

In conclusion, while you can get a lot of useful information on a travel board, never trust the experiences and the opinions posted here fully until you get your bag tag printed, at the origin airport, on your departure day.

If you want to make sure that everything is set up correctly, you can always perform a short check. Have a look at the luggage tag that the agent at the airport will attach to your suitcase or bag. It carries all the relevant information. And for you, the most important are the airport codes.

These codes will tell you exactly where the bags are heading for. If IATA codes are a closed book to you, check out this website. Memorize the codes that you’d like to see on your tag. If you see that the labels only show the IATA code of the first airport you’ll apporach, you can be sure that the agent didn’t check through your bags to the final destination.