it is really easy to tell if they are repro or 
not.

1. they are totally perfect, and color tone is 
different, from regular cels, if you have one on 
your hand you will be able to tell for sure.

2. check studio pierrot site, they have the 
images of the repro on their site, only a few 
selected shots are used to be made of 
reproduction cel, take a look at image below, 
this is a reproduction cel, and I have seen some 
Yu Yu Hakusho cels been sold on dealer site 
without mention it was reproduction cel.

3. repro come in a frame, and cel has no sketch 
or sequence #, some studio production cels also 
come with green frame, but they have sketch and 
sequence #.

4. I have saved all the images of the 
reproduction cels made by studio pierrot, so if 
you have a doubt on a cel in the future, just 
post here, I will post the sample image of the 
reproduction cel from studio pierrot`s web site.

again it is really not to hard to tell in person, 
but sometimes it is kind of hard to tell by just 
looking at the scan.