I saw most repros with the studio stamp on the 
right bottom corner of the cel to indicate it is 
a repro (official repro) from that studio.

Whilst, some others will state e.g. "15/200" to 
indicate this is the 15th repro of the total of 
200 repros they made.

Some repros do not carry backgrounds. If they 
have, MOST PROBABLY, it is a laser copy 
background.

Others come with certificates. Some do not. The 
only way to tell if it is an official repro is 
like differentiating a fake and an original 
production cel. Well, I thought the same way 
applies. Just my 2-cents.