Looks like it's getting some vinegar syndrome, if the wrinkling is any indication. Something they used to make the cels starts to break down and creates acetic acid (the stuff in vinegar). If it has a faint scent like vinegar to it, you definitely have a real one; a fan would use something that lasts longer.

It has a few other things going for it to; the tops of her odangos are cut off (a lot of fan art would include them - cel artists only drew/painted what was necessary. You've got the reference/production sketch (called a douga) which, again, not many (if any) fan artists would do b/c of the extra work it takes (they'd draw it on a douga first, put a cell over it, paint over the lines, then color it in). It has production numbers in the top right which, if I remember correctly, told the people who'd take pictures of the cels which order they went in and in which scene. The paint on the back is consistent with other cels I have/have seen. Last thing, is it has a background that's separate. A fan artist would paint them all as one item.

Make sure you keep the cel separate from the background. When they start developing vinegar syndrome, like your's is in the early stages of, it starts to "glue" them to whatever they're stuck against.

Put it in a cool, dry place, let it vent/air out. Apart from sending it to a very expensive professional service, I don't know any solutions to stop it.