I have yet to see a fan reproduction, or "fake", 
that looks real enough to fool me. When you 
collect a series for long enough, things tend to 
jump out at you. Sometimes the shot is just too 
good, you know? If a cel looks pristine, that is 
a big no no. Every cel has at least a little wear 
on it. The occasional scratch, fingerprints, some 
dust here and there. Sometimes, you come across a 
cel where the character is in a familiar pose, 
like Vegita folding his arms, but his body is 
just a little out of proportion. It's little 
things like that which let you know. The biggest 
hint, is the paint and line drawing. No matter 
how good, it is very diffcult for an artist to 
exactly recreate the paint sceme of a scene, or 
draw the lines with absolute precision. Just go 
with your gut and you should be fine. When in 
doubt, ask an expert or post on the forum like 
Cres says. I must ask...What the hell are you 
spending $500+ on!? I cannot conceive spending 
that much on a cel unless it was Mononoke Hime or 
something like that. They must be awesome shots.