I have actually started collecting cels no more
than 2 months ago.  So I wouldn't be able to spot
a fake one from a real one . . . yet (I have
collected many Vintage Barbies too and know how to
spot a counterfeit by the types of paint used for
the faces or how the vinyl smells, but that is a
different category!).
The most expensive cel I bought was $280 from Ebay
and it had the studio Periot seal on it which
shows that it is authentic.  The other cels I
bought that were $100 (give or take) were from an
online store which specializes in cels (Anime Taro).  

I almost paid $575 for a "Fluffy" cel, but was
skeptical as to whether or not it is fake.  But,
what I would do for expensive cels, is request a
Certificate of Authenticity from the dealer/studio
(or a reputable source) that indicates it is real.
   That is what I did for most of my collections
(have well over 3000 collectible items!).

J