I'd certainly agree with the colors--- and really 
want to latch onto the part about the lines.

Unless someone's a very experienced artist, or 
who has formal training, an animator's lines are 
going to be a lot smoother and more flowing than 
a fan-artist's. Unless you're extremely 
comfortable with your medium, I'd think the 
natural instinct would be for short, choppy lines 
with a little squiggle. It's primarily visible 
freehand, although if someone's a tracer, it may 
still be obvious-- if not in the ink lines, 
perhaps in the borders between the different 
gradients of shading and highlighting paint. But 
animators have experience and steady hands, so 
their ink/paint lines don't have that problem. 
You can really see the line difference in pencil 
sketches, although both may be noticable in a 
good-sized image of the cel. In addition, are all 
the inked lines the same thickness, or did 
someone actually change their marking instrument? 
Their shoulder-- their jawline-- their mouth-- 
their hair-- their eyes? 

But it's important to be familiar with what you 
collect. Honest people will be nice, and say very 
cleary, "Hey! This is a fancel!" It's probably 
only with the higher-dollar series that you'll 
have to be careful about people trying to fake 
cels on purpose. But in general, know enough 
about the series your collecting to know what 
you're getting, if you're concerned about 
accidentally getting a fancel. ^_^

-Cres