Cheating is a shockingly common occurrence in gay relationships (all relationships, really). Gay guys cheat for a variety of reasons. Sometimes they want out of the relationship but are too cowardly to just break up. Sometimes they just feel trapped and want a fling without getting caught. Sometimes they're just incredibly stupid and drunk. The reasons aren't terribly important because you feel heartbroken and betrayed and a ton of other awful stuff. Being cheated on is the worst. You don't want it to go on any longer than it has to so here are the biggest signs someone is cheating.
First up is sudden changes in schedule and appearance. If your boyfriend is working out more, grooming more, and generally taking better care of himself for unexplained reasons chances are good he's stepping out on you. He wants to impress his new lover and make the gay sex sizzle. With no other motivation for the physical changes it's hard to assume anything but cheating. Along those same lines if his schedule suddenly changes or gets unpredictable it's a good sign something hinky is going on. If he waves off your questions about the changed schedule by saying something about work but not really explaining it that's a really bad sign.
If your boyfriend starts being secretive with his cell phone and clears his browser history on a daily basis you should consider that he might be cheating. If he texts all the time and won't tell you who he's texting or is clearly lying (lots of smiles and laughter on his part and then he tells you he's texting someone from work is a good sign of a lie) then he's probably texting his new lover. You can try to track that sort of stuff or you can just end the relationship and move on.
Systemic cheating (more than just a one night stand) is almost always the sign of a relationship that is already on the decline. However, when the cheating begins you'll often notice a marked decline in the intimate time the two of you spend together. In a truly bizarre turn that's often because your boyfriend feels guilty about cheating on the person he's cheating with! It's also often because he feels guilty about cheating on you and doesn't want to be close because the guilt is too painful. Either way you'll likely notice more stilted conversations, less gay sex in the relationship, etc.
A spike in defensive reactions when you inquire about your partner's whereabouts and plans is a sign that he might be cheating. If he wants to keep parts of his life secret then he doesn't want you to know what's going on and that's never a good sign. In truth, most of the time you know that your partner is cheating before you have to do any of this investigating. Cheating is a sign of a relationship that has atrophied and is darn near dead. The key is to fix it before it gets to that point or end it and move on the next guy.