There is no real indicator on the cel itself; 
it's more of a matter of knowing the series, or 
being able to mentally reconstruct how the layout 
is... laid out... just by looking at the scan.

Bank cels are usually part of an easily 
recognizable sequence that's used multiple 
times... ie, in SM fandom, it might be whenever 
Sailormoon whips into her transformation 
sequence, or starts to hurl her tiara, or spins 
around twirling her Cutie Moon Rod. ;o) Rather 
than re-animating, they use stock footage. That's 
different from, say, filming the A-3 cel five 
times in the same sequence (ie, someone 
talking/running), or a flashback, or something 
like that. But in order to know that it's a bank 
cel, you need to be familiar with the series.

Book cels are a little more difficult. Both of 
these are still in my "soon" pile. I expect the 
one on the left to be a book cel. It may or may 
not be, but the chances are good. I don't think 
the one on the right is, but there's a sliver of 
a chance that it might turn out to be... I've 
been watching that scene, and I can't tell if the 
rocks at the bottom are a foreground, or if 
they're part of the background. From how much of 
her foot I can see in the scan, versus how much 
of her foot I can see on tv, chances are leaning 
towards it being book... but I won't know until I 
see it. ^_^ But it's the quality of having at 
least a foreground plus a background, and various 
layers of cels sandwiched in between, that makes 
it a book cel rather than just a multi-layer 
setup.

-Cres