In order to know whether a Japanese person is really saying yes or no, you need to study their behaviour quite closely. If a person is clearly trying hard to be nice, they are most likely avoiding saying ‘no’.
The reason for this is that they do not want to hurt your feelings or pride.
A Japanese person will continue to use expressions such as ‘why not’, ‘we’ll see’ and so on, thus avoiding a definite answer.
In other words, if you do not get an overt and very definite ‘yes’ from a Japanese person, the answer is probably ‘no’.