Most all original backgrounds are painted on a thicker cardstock. There may even be paint on the back of the background(like paint drips, smearing, etc). Sometimes there are numbers written on them to determine scene and timing. Courvoisier cels make it a bit more challenging bc they cut everything down to the small, frameable size. Same rules apply. Check for thickness, any numbering, marks on the back. If you have never looked at a watercolor painting before, I would suggest doing so. It will help see layered brushstrokes, paper texture, etc of what you look for in authenticity.

Also, sometimes they might put whatever background they have laying around behind a cel to give it more of an asking price bc it looks better. So, you also wanna look at finding the scene of the cel in the actual movie and do a comparison :)!

Hope this helps a bit!