I set out to find the answer...and stumbled on a
thread that was endless (it got too long for me
this late at night)...so I'm just going to past
this part of the thread.  Hopefully, someone who
already knows something about this (yann?) can
provide more insight.  

http://www.eu.animanga.com/cels/book/archive1/p1226.html

" Knowing what copies are around and the
whereabout of original is obviously of big help.
For example, AIC printed a catalog of the hanken
they sold during the GoFa exhibit. All those
hanken are coming with a certificate from AIC. So
if you see one of them but without the
certificate, it's a copy... But that's a special
case and many time, there isn't such
information..." (yann)

but I'm not sure if the Ah my goddess hankens were
sold during the GoFa exhibit or not...


^^
joyka
p.s.  Love your new washu cel