Some of my Studio Pierrot cels have the 1998 20th 
Anniversary seal on them, but that's about the 
closest thing to anything that would accompany a 
cel to establish it as being authentic. It's 
certainly not something to judge a cel's 
authenticity upon-- and I haven't figured out the 
pattern for what got marked and what didn't.

Wasn't there something about the Vampire Hunter D 
movie cels coming with certain certificates or 
something, as long as you got it from a specific 
person?

But things like that are going to be the rare, 
rare exceptions, rather than the rule.