If you ever have a question on an anime cel on a 
dealer's site, or an auction...You should always 
feel free to ask the question here. 
Unfortunately, some people seem to not agree with 
posting auction links on this forum which is 
unfortunate...People looking for help and advice 
get flamed...No wonder people sometimes have a 
problem with this board....BUT...

Anyone ever needinghelp can always feel free to 
contact me.  I'd be more than happy to help 
anyone out with authenticity question.  I'm not 
the end all be all when it comes to this, and if 
I'm unsure of my judgement...I'd be more than 
happy to point you in a good direction.

Nick's Site(Anime Chaos) has a great Yahoo! Japan 
board.  That may be the best place to start with 
your Yahoo! Japan questions.

As far as writing a list on how to authenticate a 
cel properly...Well...I can't.  Being able to do 
that properly requires a good amount of 
knowledge, resources, and time in collecting.  
Usually cel authentication questions come about 
Oh! My Goddess(Ah! Megami-sama) cels.  That's 
where a lot of that looking for the proper 
shading and coloring next to the screencap came 
in.  But sometimes colors and shading are 
digitally touched up...etc...But a lot of 
reproduced images outside of japan were made an 
thrown into the market with AMG.  With any anime, 
if you have access to a screencap of your 
cel...It's a good place to start.  And if you 
have your doubts after that post your questions 
here or to a fellow cel collector who is willing 
to help out =).

If you or anyone else has any other 
questions...You can feel free to post them here, 
or e-mail me =)!  

Hope this helps!