text: how can you tell if your significant other is cheating on you? we've been together for a year and a half now. i am happy and i think he is too. i think he has been with someone else, but im not so sure. he's cheating on you if you've noticed him acting strange or suspect around you, arguing for no reason at all, stopped spending as much time with you as he use to, or when he is always busy doing something that he's never done before this problem occured.