>  How can I tell if a cel book's pages are acid-free if it
>is not labeled on the cover? 

Disclaimer:  I'm not an expert on this subject, but here's my input.

   I don't know if they label cel books as acid free.  The 
main thing that I consider important is that the clear plastic 
pages do not "stick" to the cels.  Often, many types of plastic 
seem to stick (if you look at it, there are spots where it 
looks like there is a bit of liquid where the two pieces of 
plastic touch.  It looks like some chemical, but when you 
separate the two sheets, it disappears.  This also happens 
with photographs and slides.  

>And usually, when I get a clearbook
>/celbook, there's a yellow piece of paper serving as the 
>in-betweener for a cel book's page.

   I'd say that the sheet is most likely not acid free.  But 
then, I don't think the pencil drawings are done on acid free 
paper either, are they?

>  Where can I get a piece of plastic to put on the back of the
>cel as to prevent it from sticking to the cel book? But in using
>this piece of plastic, the cel's paint will get stuck to the plastic
>so that doesn't work very well either. Any other options?
  
    I'd suggest just using a piece of acid free paper behind 
cels that you don't mind losing, should it ever stick.  Other 
suggestions have been some type of "silk paper", which should 
work too.   

>  And do people usually store their cels by size or by series
>or some other way?

   I store them first by how much I like them, and then by 
series...  Of course, oversize cels go where ever you can 
fit them.