Well it's easy to tell if a background is 
production or a laser copy... but unless you know 
what episode the cel is from and check the 
background, there's usually no way to know.  
Maison Ikkoku cels and backgrounds are nice in 
that most of them (especially after the first 
season) have epidoe numbers  and sequence number 
on them.  If the numbers match, so does the cel 
and background, AND you know what episode to look 
for it in.  Unfortunately most cels are not so 
specific.  Matching numbers can sometimes help... 
but say something says it's from episode 10... 
well how many series out there have an episode 10? 
 And it migth be from a different sequence.  The 
only real way to know is to watch the anime and 
find your cel, and see if it has te same 
background.

Tifa