Alright I'll take a stab at this.

Check the texture of paint on the back. Cel vinyl 
is smooth and matte if acrylic was used it will 
be glossier and the texture will probably be more 
uneaven.

if the series is old, you will most likely be 
able to see the xerox lines showing through the 
paint on the back.

Check for some line fading - people usually don't 
try to fake that.

Check the color against cels from the same 
sequence.  A lot of color adjustment is done in 
post production and fakes are usually made to 
look like it didi on screen not nessisarily how 
it looked as a cel.

Check the line work.  A lot of fan cel artists 
ink there lines from the front not the back 
because its easier. if you look at the front of 
the cel in the light and you can see the lines 
shimmer differently than the normal gloss of the 
plastic its probably marker on the front of the 
cel not xerox on the back.

Smell the cel (if you can examine it) cels have a 
sort of acidic, battery like smell (god I ove 
that smell!) fan cels usually dont.

Check for condition if the cel is absolutly 
pristine and its a really old series that might 
not be right, of corse if the cel had one owner 
who kept it in pristine condition all bets are 
off.

See if the sequence numbers are written in the 
same way that that studio uses, if someone else 
has a cel from that sequence see if the numbers 
are consistant  i.e.  the real cel says B11 and 
C11 when the fake is a single layer A28.

Most fan cels are single layer.  If anyone else 
has one see if it has seperate layers for the 
hair, mouth other character ect...

Some studios stamp seals on there cels find out 
wich ones and if yours should have one.

Imperfections like staple wholes usually aren't 
faked.

Fan cels usually have chunkier, uglier line work 
being that there made so carefully peoples hands 
tend to shake more. And there not copied from a 
thin smoothe pencil drawing.  Ink is harder to 
get a perfect line with.

If there are any lines showing at the edges and 
you can disolve it with a cotton swab and some 
rubbing alcohol its marker not xerox.

Some Fan cel artists make a genga first to make 
the cel by - don't go off of that alone.


At the end of the day a good and crafty fan cel 
artist who knows there stuff could fake a lot of 
this stuff.  And, at the end of the day there are 
always odd cels that are perfectly legit that for 
some reason are different than there sequence 
mates.
That said if your really unsure ask around, check 
the dealer as other peolpe suggested and if you 
have a really bad feeling about it don't bother.  
There will always be another available 
sometime!   

(unless its an A1 End then your skrewed unless 
the persone decides to sell later ^_^)

Hope this helps!  The best way to learn to spot 
fakes is to look at alot of originals and fan 
cels and learn to feel the differences.
Morgan