Does anyone know of an online tool where one can practice grading coins, and then see the real answer?
Actually, this is one of the best online tools if you use the search box above in the blue menu bar. Rlm has quite a few of posts similar to this one http://www.cointalk.com/t193872/ Jim
I agree with Jim. CT is probably the best opportunity for you to do that. I've put up a couple ones too. Most people put up coins they've had graded, and then at the end of the available time to "guess the grade" they show the actual grade the coin got. It's really a good thing.