Even though there aren't many certificates out 
there (i'm assuming). Some studios do have ways 
of labling that prove (as much as a cert. would) 
if they are authentic or not.

AIC has stamped thier cels. 

Studio Pierrot has little stickers on theirs (at 
least the 20th anniversary releases do...)

Anime Link has stamped all of thier VHD cels (for 
more legal release reasons than fraud.. but it 
helps).

And a matching douga is pretty good proof as 
well, if you think about it.

Of course, with a lot of effort, these can be 
fake as well, but just as much is to say about 
paper certs.