All the fake cel's i have ever seen don't come 
with a sketch.  This is probably because the 
perpatrator either used the real sketch several 
times to redublicate the cel, or used a drawing 
he made to reduplicate the cel's.  Plus paper 
shows age much more than paper.  I can tell some 
of my cel's sketches are old.  It would be a 
giveaway if the sketch was gleeming white.

If i got the sketch with it and i was fake, i 
don't know how angry i would be, because that 
would make it original artwork, and not 
duplicated.