Hmmmmm... I always print the cel image on best 
printing settings and compare it closely to the 
scene in the show. Meaning, take the tape and 
pause it. Compare each single line! 
A big problem are how colors sometimes display. 
For example, when Zelgadiss fought Glavoss in 
Try, he was very pale. The scan of that cel made 
the colors more vibrant and baka me thought it 
was fake. Scanners sometimes screw up, you know. 
But they do NOT screw up lines and stuff, unless 
you move the cel during the scanning process...
Note that you might see more of the chara on the 
cel than in the show. Sometimes they draw the 
chara from head to feet, but they show him 
beginning on the ankles.

So, always compare:
-each single strand of hair
-each line (may fade on cel, but the direction 
will never change, neither do lenght, 
thickness,...)
-to how many degrees eyebrows are turned (YES, 
each tiniest mistake/difference betrays a fake), 
etc.
-Very, VERY beautiful main character shots... if 
a seller as a lot of them, it IS of course 
possible he was just lucky, but if you overdo it 
in your collection of perfect cels, you may get 
in suspicion. Like me, I (try to) always only 
draw beautiful fanarts. A faker would never get 
the idea to copy a less distinctive shot. 
-Do NOT watch the scene with compressed video 
files like RM. The bad quality detracts from the 
details. VHS or DVD are better, or at least 
AVI,...

MY NOODLES ARE OVERBOILING!!! GOTTA GO!!!