Some studios, such as Pierrot and AIC,
 imprint cels with their seal to prove its 
authenticity.  However, there are always AIC
 and Pierrot production cels without the imprint
 so...^_^;;;;  It's no big thing really...the only cels 
that you 
have to watch out for to see if it's fake or not 
are
Ah! My Goddess cels in my opinion.  For most 
of the
other reproduction cels, you can tell (they are
serial numbered, etc).

Gaudiora