This is an old David Blaine trick.

Effect: You have a spectator drop three dice into a glass of water and have them look through the bottom of the glass and add up the total then have them set the glass back down. Then dip your fingers in the water and rub them on your forehead and you magically know the total.

How-2: All you do is add up the top faces of the dice and subtract it from 21, which will give you their total, easy!

NOTE FROM ICHIGO: If your glass is too small to fit three dice (or if your dice are too big), then you can just use two dice and subtract the total from 14 instead of 21.