How do you tell if a guy is cheating on you?

Well from personal expirence I would say ask him a lot of different questions and see if he can keep up, then re ask the same questions. If he can't answer the same question the first way he did then you might want to worry. Also if he calls you or anothe female you know the wrong name then you have him right there.

And for something more realistic , working long hours, not being where he said he would be , smelling of some other womans perfume , not wanting sex as much , or more than usual, doing new tricks in bed , making a dash for the shower the second he gets home , any unexplained absences , seeming like he can't wait to get out of the house or away from you. Don't go making wild accusations. Eyes and ears open , mouth shut , til you have some concrete proof.