Before you start, get his modifiers.
What's your attack without dice? hide without dice? etc, etc.
If his attack with a shortbow at fourth level is +2 (BAB) +4 (Dex) +1 (focus) +1 (MW) so that's 8 right there. If you know his attack modifier with his bow is usually an 8, and you see the rolls (and you should ALWAYS see the player's rolls) you know he can't score above a 28 or so on an attack. He could hit above an AC20 so long as he rolls over a 12. Add in bless and flanks and all that, and he could be swinging like a champ.
I guess my main advice is to learn your characters. Know what they are capable of doing, and what is reasonable/possible for them. If you are blind, everything will surprise you and seem out of place. Once you can open your eyes, you'll be a better judge if there is foul play or not.
If this guy is truely a munchkin, and is good at it, he's probably not cheating...it just seems that way because you don't understand everything that's going on...and a DM needs to understand what's going on in his campaign.
I collect character sheets at the end of every session. I can go over them any time I wish, and there are no fears of character 'alterations'...IYKWIMAITYD.