No, in the case of the LU cels, the scans of the 
sketches were actually nicer than many cel 
sketches I've seen here.  And if you want to make 
more copies from your sketch, there's a great 
solution: photocopy.  You don't need an authentic 
sketch to remake a cel, just knowing where the 
lines and colors go is more than adequate.

Sorry, but TONS of cels get sold without 
sketches, and the vast majority of them are 
real.  If you want to be able to tell better 
which is fake and which is real, you're going to 
have to go the old fashioned way with screencaps 
and a lot of time to sit and examine and examine 
again.