If you are ever worry about if the cel is fake or 
not, you can always locate the exact scene on dvd 
or video and compare, definitely worth the extra 
trouble if the cel is expensive, it is impossible 
to make two cels that match 100%, even on the 
official hand painted reproductions cels, which 
are painted by professionals, even these has 
slight difference in the highlight lines, no two 
hand painted cels will ever matching 100%.

In most cases fake cel looks horrible, with 
exception of studio made official reproduction 
cels.