I would have to agree with a lot of what Yann 
said.  

Usually a hanken cel is bigger than the average 
cel.  And with hanken most of the image is 
painted...The wouldn't be leaving the top of his 
head unfinished, and maybe even the right side of 
his body.  If there's registration holes that may 
be another sign of it not being a hanken cel.  
BUT...On the other end of that comment...there 
are cels that are used in the actual production 
of a series, and then used for other products, 
like video jackets, poster, art books, etc. 
Saiyuki wasn't done with cels...It was all 
CG...So it there's registration holes on the top 
I'd be a little weary unless you get it's 
authenticity verified.

The best thing to do is e-mail Animegame and ask 
them more about the cel.  Like...What is that 
labeling at the top of the cel?  Are there 
registration holes?  Where did you purchase the 
cel from?  Is there an authenticity certificate 
with the cel?  If not, how do you know it is a 
hanken cel?  They should be more than happy to 
answer these questions for you^^'!

Hope this helps!