We have one player in our group that everyone is pretty much convinced is cheating at die rolls. He's pretty good at it, though, and it is hard to catch him red-handed. The two tell-tale signs that he's most likely fudging a roll:
1) Rolls his dice and snatches them up before anyone can see the result. Basically, his hand follows the roll and picks the die up the isntant it stops rolling. This way he can just pick any number that he feels like. Oddly enough, he usually picks numbers from 15 to 20.
2) Long (and I mean LOOONG) pauses after rolling as he adds up his bonuses. I mean, seriously, if you swing your sword at an orc, how come it takes almost a minute to add your +3 attack bonus to whatever you rolled in order to get that 22? Basically, we all feel he's spending that time debating on what total would be high enough, but not too high to appear like he is cheating. Of course, the fact that it takes a guy (who is actually very good in math) that long to do simple addition is the dead giveaway.
As for dealing with the problem. This is the tough part. We've already lost a player in the past because they were tired of this guy's cheating. DMs have tried talking to him, but nothing seems to stick. He'll stop for a couple of weeks, but then the fudging resumes. What makes the situation worse, though, is that this guy is my roomate and we game at our apartment. And I know that kicking him from the group is going to cause nothing but problems , as friends and roomates.
So instead we all just let him go about cheating, each game getting more and more annoyed at this juvenile behaviour. He is always the best combatant, gets the best skill rolls, always wins initiative. It's frustrating mostly because this means the other players don't feel like they matter. I hate that this occurs. Almost anyone else would be kicked to the curb, but since we've been friends and roomates for a long time, it seems like a trivial matter to ruin that friendship over.