This is a good question ;-) and I thought about 
it for myself in the past, too!

I think if you are collecting for some years, you 
get a "feeling" what is okay and what is not 
okay. 
But - as said before - it is good to know the 
seller and it is good to get some idea about 
prices of the series. If it sounds to good to be 
true, it could be a fake ... If you are not 
really sure about a cel on auction, better let it 
go (especially if it is high priced) even there 
is the danger to risk loosing a great deal (that 
are my 2 cents *gg*). I think a big problem are 
duplication cels and repro cels, that often look 
like as they could be original (especially if the 
cel is scanned and shown in a way that you cannot 
see if it got a number or not).
If you collect cels from your favorite series, it 
also should be no problem, because you know how 
it looks like.
And I think if there are fake cels, they are from 
series that are high priced, because it is not 
worth fake a cheap cel ^^

Usually cels should have registration holes and a 
registration number. But old cels are often cut 
at the top and have no registration holes or 
numbers. But if there is a douga or genga, this 
is a good indication for authenticity.

And look around, look into galleries and sites 
from dealers, to get a feeling what could be real 
and what is not. If you are in doubt, better do 
not buy it. Of course, this is also a question of 
price - if the cel has a price of 1000 Yen and I 
am not really sure, I don't mind and take the 
risk *g*

And at least you can come to this forum and ask 
the cel colleagues ^^
(BTW - sorry for bad English!)