pls friends, i want to tel me how to know a sincere man.

impossible, you just have to go along and hope that whatever he tells you is the truth. a lie will always come out one day or another so time will only tell. do no rush into anything that you are not certain of, and if you have any doubts then "investigate".
know that some people are VERY good at being dishonest and hiding the truth.

yes, thats it. i always believe everytin he says , but at the end they all turn a mirage.

it means that you want to believe what they are saying so much that you dont use your head instead.
believe what you see, believe what you feel, dont just believe anything that comes out of any horses mouth.
if you accept the fact that most people lie, either it be the small white lie or the absurd non sense then you will always be on your guard.
also (dont get me wrong) being desperate will make some people ignore what is the obvious for the sake of not being alone.
use your head and time to flush out all liars. dont rush into anything.