How can you find out that your man has been cheating on you in a long distance relationship my man an I have been apart for a 1yr and half?

Surprise visit him and if he's doing the dirty, you'll catch him out!

Orrrrrrr.... Get one of your friends to call him and chat him up and see if he bites!

If you and he have been apart for a year and a half he is not your man, there he cannot be cheating. A separation that long would almost certainly he has found some other interest.