I'm glad there's a counter-question to How can you tell if your girlfriend is cheating on you.

Your boyfriend could be cheating if he's staying out late with "the guys", giving vague answers to your questions, showing less interest in you ... But, on the other hand, these things could mean he's just being a guy.

If your boyfriend has suspicious behavior, you can look out for evidence he's cheating.  If there's more smoke, there's more chance of a fire.  Even if it's not cheating, it's a problem that you should talk about together.