No, like for normal cels, there isn't any easy way to
tell if it's a real one or fake.

If you can find for which produce an hanken was used,
you can compare the cel with the produce : it should
be _exactly_ the same. For a regular cel, you can only
check with a picture on your TV and there is distortions
and the resolution is poor. A poster, LD cover... doesn't
have such distortion and is of much better quality. So
easier to compare.

Most hanken cels are usualy much bigger than a production
cel.

As for an EVA hanken : I realy don't think that you can
find any legit one : Gainax already can't release cels
from the anime. Hanken from Eva would not belong to Gainax
but to the company that produced the good. And that
company won't give aways an hanken... (they already don't
want that Gainax release production cels so...)

Cheers,
Yann Stettler