One of my players is really good at sleight of hand tricks.
Should I assume that she's cheating when she rolls really well?
Buy a dice-tower. Buy a load of cassino dice and don't let her use her own.
Does she seem like the type of person who would reasonably cheat at a group activity just for the sake of imaginary points? Does she consistently roll above and beyond what she theoretically should, and always during crucial moments? Is she extremely competitive and has to constantly win?
If not, you're being a little too paranoid about it. Believe it or not, most people aren't going to cheat at a group activity like RPG's unless they're playing in a competitive environment (which you really shouldn't be anyway, but I digress).
Worst case, just make everybody roll out in the open (which everybody but the GM should do anyway).
I have a player that cheats on rolls all the time. I don't do shit about it. Why? There are plenty of ways to fuck him over in-game he can't cheat his way out of, and if he has to cheat to have fun and can't handle failure that's his hangup, not mine. I don't need to pray he decides not to cheat to kill his character off, if I were so inclined. I just let him do what he does and carry on, because ultimately, we all have fun.
Have her teach you how to do some of those tricks; they'll get you laid, OP.
Dice boot or dice tower (google, amazon and ebay, or diy), make everyone use it, keep it in the center of the table (or other easily visible spot), done. No drama from accusing someone of cheating, no real chance of cheating. The only person who should be rolling out of sight is the GM. The only time the GM rolls in plain view is when it is important enough of a situation that everyone should see just how the dice fell.
If you understand how sleight of hand tricks work, then it doesn't matter how good a player gets at them. Pick up any dice magic tricks book, and it won't take long for you to call out the players bullshit.
Cut her hands off like you would any cheat. Make her learn the consequences.
Then rape her and piss on her severed hands
Fudging their holes you say?