This is a difficult and delicate situation. The first thing to do is to sit down by yourself, clear your mind and emotions, and be rational. Once you get your friends or family involved, things tend to spiral out of control. In their effort to be supportive, they may say things about him that they don’t really mean, which can be awkward later.
Now think: Why do you suspect him of cheating? Is it something you saw or heard? Or is it a conclusion you jumped to because you’re insecure or because he’s out a lot? Be honest with yourself. Suspicions will make you act differently toward him, which can undermine a good relationship.
Pay attention to his habits and body language. Has his routine changed drastically? Is he out much later and more often? Does he avert his glance when he tells you where he’s going or where he’s been? Does he seem more distant and detached in romantic situations? Set up little tests. When he says he’s meeting Jake at the coffee house, tell him maybe you’ll drop in for a minute when you’re driving by. See what his reaction is.
The main thing to remember is to not obsess about it. Check his Facebook page and Tweets and those of his friends. Look for posts and photos that might explain his whereabouts — for better or worse — and be objective. This is all public information. Be careful about taking his phone and looking for texts and call logs, as this may be seen as a betrayal.
If the doubts persist, you may have to confront him. Don’t do it when he’s ready to walk out the door. Wait for a safe, warm, comfortable setting. And don’t accuse but ask.