How do you know if a Japanese person means no when they say yes?

While this phenomenon exists, it's not as big a problem as you think, and it affects primarily relationships with people you already know. If you ask a complete stranger for directions, they will say "no" or direct you somewhere else if they can't help you, and if they go out of their way to help you they're doing it entirely voluntarily. Sure, you might get a "I'm not sure, but I think it's over there (vague wave of hand)" response if they don't know, or a display of teeth-sucking hesitation where they stand around repeating the name of the place for a while ("Anpanmanji Temple, sou ka, hmm, so you want to go to Anpanmanji, sore wa chotto ne..."), hoping you'll take the hint, but in both cases it'll be pretty obvious they don't actually know, so you can make your apologies and ask somebody else.

Where the yes-but-actually-no thing can happen, though, is with people you already know on some level, especially if they feel an obligation of some kind to you. For example, if you're invited to somebody's house or picked up while hitchhiking, you're now the guest of your host and he's responsible for your comfort and well-being. So if you happen to say off the cuff that you really like sushi, or that you're actually going to Town X 50km away, your host may well offer to go get some sushi (and throw away the dinner his wife spent the day preparing) or take you to that town (and miss the baseball game he was planning to catch).

The traditional Japanese cure for this is simple: refuse the first two offers.

• Oh, you're going to Town X? I can take you there if you want.
• No no, that's fine, just drop me off at Y and I'll go there tomorrow.
• You sure? I can take you to X if you want, it's not far away.
• No, please don't worry about it, you're already been much too kind.

If they repeat the offer for a third time, they're being serious and you're expected to accept the offer. If they don't, you've just avoided imposing an obligation on them.

Japanese people don't really say "yes" when they mean "no". What they say is "it's very difficult" or "it's unlikely". The only time they will say "yes" is if they think you are not really asking them something.

For example if you invite someone to your house and they say "yes" without asking for details of the exact time and your address it is probably because they assumed you were not really inviting them to your house. It's like you are saying "you are welcome to visit me" but not actually setting up a gathering. So in this case it is simply a question of the way the Japanese language works, in that it tends not to differentiate between saying something would theoretically be okay at some indeterminate point in the future and actually arranging for that thing to happen.

The key is to understand what you are asking, then the answer will make sense.

I've found that a 'no' usually comes in a roundabout way - as an alternative plan or delay in answering, or occasionally a rub of the forehead followed by the phrase "?????..." ("chotto...", colloquially "well...").

In business settings 'yes' is usually used to indicate "I am listening". When you need a clear answer, leave the request open so time is allowed to discuss and come back to you - often times the answer is known, but if it's a 'no' you'll probably find out the next day via email.

Generally though the refusal to say 'no' is just a mechanism to avoid confrontation or an embarrassing situation, or to 'save face'. Instead, 'shape' the request so that neither can happen. If you can't do that and you're not friends, probably best not to ask.

As commenters have said though, if someone goes out of their way to help you they're probably just nice people - don't be too paranoid :). If it's someone you know and is a big ask, a small gift or token of your appreciation helps too (though expect it to be unnecessarily reciprocated!)

There are other good answers already but I just want to focus on an error in the way the question is asked.

A Japanese person doesn't (generally) mean "no" when they say "yes", which is the premise of the question.

This mistakes what they are saying when they say "yes". They are saying "yes, I hear you, and I will process your question" or something similar to that.

It is only you who thought they meant "no" when they said "yes". Actually they didn't give you an answer yet: that is the important thing to realise.

It is actually similar in this respect to the side to side headshake of Indian culture.

When you hear "yes" you aren't faced with the question "did they mean 'yes' or did they mean 'no'?".

You are faced with the situation where you now know that you will need to do more in the relationship to find out what the answer is going to be. And, in all likelihood, whether you like it or not, you have probably just discovered that you won't be able to get the actual answer in this conversation: it isn't time yet.

There's a really fundamental cultural reason why it is this way: the importance of "politeness" in Japanese culture. It is simply impolite to say "no" directly, and impolite is bad.

(A trivial example of this is if a waiter asks you if you would like some more. The safe polite thing to say is "I am fine". This avoids saying "no".)

In doing business in Japan, I've found that one way to explore whether something is a good idea for a third person is to talk about the politeness of it. Instead of saying "Can we haggle?", I would ask "Would it be impolite to offer them a little less". Our Japanese agent will then easily be able to explain that yes it would be impolite, or to say that it will be OK.

This is a more difficult task in a face to face, one to one situation. In answer to your question "is a way to ask them for help so that you can give them the opportunity to decline without them feeling bad" I can only say that you need to find a way where giving the answer they are comfortable would not appear impolite on their part.