I think you can't really tell based on the cel 
itself, but once you have, let's say, 5 cels of 
the same series, you can recognize them by 
checking several things:
- that a given color is exactly the same on all 
of them (for example the paints that were used by 
the Toei were not those used by most other 
studios)
- the thickness of the lines, supposed to be 
homogeneous for a given series (or rather for a 
given episode of a series. I mostly collect cels 
from episodes 57 and 67 of Saint Seiya, and they 
don't look like those from episode 62 for 
example, as another team dealt with this one)
- the line fading. If a cel supposed to be 20 
years old cel is to good condition, you can begin 
wondering what's going on.
- the sheet of plastic itself: size, thickness, 
and so on. If you had the right equipment you 
could even have a look at the grade: molecular 
weight distribution, optical properties... but 
this is the materials scientist talking here ;)
- you can even compare the writing of the 
sequence and cut number. It is stricly the same 
on all my cels from episode 67 of Saint Seiya.

That's a lot of way of making the difference 
between real and fake, but I remain careful as I 
believe japanese animation fake cels are not that 
rare at all. If I have any doubt about an 
auction, I do not bid at all.