Hmmm Okkkay JAO, I thunk yew need ta talk to yer boyfrund mmkay?
Firstly though the main crux of the matter revolves around what he is doing that is nice to her. That will say a LOT about the situation.
If he's opening doors for her, or helping her out with homework, or picking her up and taking her places because she doesn't have transport, it's probably harmless.
If however he's buying her gifts that tend to have a romantic touch to them, being deliberately sneaky/evasive (on phone or in person) or just generally making excuses why he can't be with you, then perhaps it's time to start worrying.
Once you've got that part sorted out, you have to be really sure that he is doing the dirty. Guys are REALLY simple (sorry guys I've ratted us out here), and will normally give out signs that a womans intuition picks up on.
I've been with my gf over 10 years now, and she can tell if I've done something stupid/funny/dumb, because (and I'm not aware of it) but my behaviour and personality changes very slightly.
Look out for those markers, because they will give you a really good indication of if there is trouble in paradise.
Eye contact is a really good place to start reading people. I normally look directly at people when talking to them, and if you are talking to someone and watching their eyes and body it can tell you a lot. Nervous scratching/fiddling/fidgeting can give away that a person is uncomfortable with a subject, or is not really interested in the conversation, whereas shifting eyes (the eyes right trick) can give away that someone is either not interested in what you have to say, is simply lieing, or is distracted by something behind you.
Another important thing to ask, is how long you've been together/known each other. It it's only a recent relationship, he may still be getting used to things in it. If it's a 5+ year relationship he might feel trapped in a bit of a "lul" and his wandering eye is wondering "what if". If you've known them a long time, have they had a history of girlfriends?
It's impossible behind a computer for us to tell you if it's something innocent or sinister, but surely if you take a few hints from above, you might be able to piece together more of the puzzle. You won't be able to tell what he's up to, but he WILL give himself away.
Oh and the golden rule, DO NOT jump the gun until you are 100% sure you've got him red handed. It'll destroy a relationship and push him towards another girl if you accuse him of doingg the dirty without decent factual information.
Hope this helps.
Regards, Dr Phil...