The best thing to do is to confront him. As I always say: without trust there is no relationship. If you confront him and he is cheating on you, you have to come up with the decision of forgive and forget, or leave for a new beginning. If he says that he is not cheating, but you still have your doubts about it, then you could either end the relationship, since there is no trust anymore, or you could believe him and hope for the best.
yeah ask him its better to flat out ask him then sneaking and trying to find out answers or signs of cheating and if that doesnt help check his profiles to see if he has been flirting with other girls dont ask just check his profile trust me if a guy is cheating he will simple show it and not care who sees or knows and if he is leave him and you will find someone better than him.
Investigate. Ask him what he does during the time you think he is cheating on you. Ask his friends, talk with him and ask all the why s possible. Ask everything you can, to him. When he explains, you verify and analize what he says. Then, well, the best answerr in this question.