There is usually an "AIC" brand (clear) on the 
top of every cel (around where the holes and 
sequence numbers are).  If the dealer can scan 
that part of the cel and show it to you, that 
would help.

If the dealer claims that there is a matching 
sketch, start getting suspicious.  AIC cels are 
not supposed to come with matching sketches; they 
either have non-matching sketches or none at all 
(it's been AIC's policy for years).

I'm also shocked by what I think is scotch tape 
on the top of the cel (at least in the scan I 
see).  Not sure what to make of that.

To be absolutely certain, you will need someone 
to identify the scene which this cel comes from 
(or at least where Belldandy is dressed like 
this).  I'm pretty that it didn't come from the 
Movie so it's either from the original OVA or is 
probably not legitimate at all.