For once I don't have to do the writing =) The following is taken from a true veteren of the anime arts Ed Noonchester. If this were Star Wars, he would probably be Yoda =)
(with better grammar)
These are his words below about the authenticity of cels, that is, fake vs real cels.
"How can you tell if a cel is real or a copy? That is, perhaps, the most frequently asked question. It is not a question about care of cels, but it is a very important question. I wish there were a simple answer for this question. I have been collecting and retailing cels for over twenty three years so I have seen it all. The reason that people make copies of cels is to make money. There is no profit in making a copy of a cel that you will sell for $10 or so. All fakes are made of very popular and expensive shows. It is more likely that a fake will be made of Evangelion than almost any other title because the show is so popular and so expensive. I have seen fakes of many titles over the years including Castle Of Cagliostro, Galaxy Express 999, Ah My Goddess, and (of course) Evangelion. Japanese studios do not make certificates to verify that a cel is real. One studio, AIC, has placed a seal on the (some) cels to verify them on such titles as El Hazard, Tenchi Muyo, and Blue Gender. A fake cel is made to sell for good money, but not enough to pay to have the pencil drawing done as well. Not all good cels come with the pencil drawing, but if the cel does then it is almost guaranteed to be legit. If a cel comes with the matched, production background then the same can be inferred. If a cel from an older show or movie shows almost no wear over the years, then you might be careful. A cel that is twenty years old will not look like it was made last week. If it looks like it was made last week, it probably was. Another way to tell would be the index number in the upper right hand corner. This is used to match the cel with the story board and the background. If it does not have one, watch out. Last, know your source. With so much being sold on E-Bay and Yahoo-Japan, it is much harder to know your seller. "
by Ed Noonchester.
Since I used his words, I feel out of courtesy, I should plug his site with a link (Not that I have ever bought any of his cels, I haven't so far.)
http://www.animeed.com/cel_index.htm