There are various ways to find that out. You can check his cell phone, ask his friends where he goes etc.
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Ask him :/
You have to be sure about that first. If you are sure that he is cheating, dump him before he does.
He's cheating on her.
If someone is cheating on you, it might as well be spelled across his face. Boys can never keep a secret and if they are, they aren't doing a very good job at it. Signs of cheating include ignoring, distracted, not focused, and he doesn't really care what you say. Those may be signs. If you can't decide, ask a friend to ask him What's Up and why he's acting strange. Good friends always help you on things like this.