If you're referring to a male partner cheating on you then simply watch his actions when he returns home. Also, if you offer sex as soon as he comes home and he turns it down, that's a good way to know. A sure fire method is to throw out some bait. The bait would be a cute girlfriend that he does not know about, or get another woman to flirt and see if he bites .. If he takes the bait, he's cheating on you.