The important part is to be educated about what 
you're buying. If you get to the point where 
you're buying a $500+ cel, you ought to *know* 
what you're buying. It's like buying a house, or 
buying a car, or buying any other art. However, 
because you can't physically look at it in person 
before you buy, it's made a bit more difficult.

The most telltale part is looking at the lines 
and the colors. Compare them to similar cels of 
the same character. Do the colors match? Are the 
lines the proper thickness? Do the lines flow 
properly? How about the borders of 
highlights/shadows? Are they placed properly, and 
are their borders good? Are they the correct 
colors, or are they off? Animators have had years 
and years of schooling and practice. People who 
aren't trained artists have much more difficulty 
in replicating the same flow. I've thought about 
taking a drafting class, just to improve my own 
art. ^_^

Find the scene. Get a screencap. Compare 
segments. Are the blank spots blank in the cel? 
Are the shades and highlights placed correctly? 
Can you even *find* the scene? Sometimes, it can 
be difficult. I have some cels that I know *have* 
to belong to a certain episode, but I still can't 
pinpoint them. ^_^ But they're of the type of cel 
that no one would ever want to fake. *g* Still, 
if you're going to be going after the high-dollar 
cels, and if there are any questions in your mind 
about it's genuine-ness, you're going to want to 
do your research before you commit.

Other things you can do is to ask the seller to 
scan the reverse side, for a scan of the sketch 
if it exists, or for a larger, clearer scan. Some 
fancel artists don't go down far enough and only 
paint the screencap itself, rather than carrying 
the lines further. Other fancel artists 
accidentally make mistakes on important but minor 
details-- hair disappearing behind something but 
not reemerging on the other side, details on a 
piece of jewelry, or mixing up which layer of 
clothing a certain patch belongs to.

Most fancel artists are up-front about their 
fancels. It's the ethical thing to do. Because 
cels are drawn by people, there's no one way to 
determine once-and-for-all that something is 
studio-produced, and not fan-produced. 
Registration holes can be faked, as can sequence 
numbers. And they *have* been faked, by some 
unethical artists. But, if you aren't sure about 
something, ask the forum for opinions. Many 
people are more than happy to ID something, give 
their opinions, screencap and compare, and so on. 
^_^

But since there's no magical formula, it all 
boils down to needing to be educated about what 
you're buying. ^_^

Good luck!
-Cres