There isn't really a good way to separate cels that I know
 of-- but you can at least clean off the unwanted paint 
flecks which may have been left in unsightly places, or by
 other layers that you don't have.  I have had good luck 
doing this with rubbing alhohol and a q-tip.  It wipes the 
paint off very well, and doesn't seem to harm the acetate
 (but you can always test on an american cel just to make 
sure ^_^)  Be sure to avoid your actual painted image 
though!!