It's not a stupid question.  

For AMG, it's sometimes difficult to tell.  You
often need to compare the cel with a screen
capture image and check for signs like wrong color
spots.  Say the color in a certain area should be
blue instead of red...stuff like that.  And AIC
also made some repro cels themselves and since
these look very much like the production cels, it
gets kinda tricky.  It's usually a safe bet that
*if* you wish to buy some AMG cel, post the image
on the forum and get people's opinion and then
decide yourself whether you really want it  or
not.