There will be vague answers out there, telling you that, well when your partner cheats on you, you just KNOW.
This may be true, but if you are taking you relationship seriously, you should also address this matter seriously. Nothing is more serious then cold hard evidence. You do what you can to get it, spy into his email, phone calls follow the guy. And when you have what you need, he can’t slease his way around it. Because you need to do one of two things, confront him, or turn a blind eye.
If you confront him, you need above mentioned cold hard evidence, and then you take it form there, keep or kick.
A blind eye, nothing to say about that. Speaks for itself.
Most importantly, don’t confront him on a vague suspicion, because it’s useless and dumb and usually ruins relationship for no reason. Also, stay away from “well meaning friends” who sometimes steer you in a very wrong direction. It’s your life, your partner, your decision. Make it with intelligence, integrity and strength.