The original trunk lids are all fiberglass with a texture to the underside of the skin. If you can see an area that's not painted, it will have a pinkish color. The only way to tell for sure if it's a real CS though is with the Marti Report. The price guide on this site is a good start, but it is a couple of years old and prices on all Mustangs have gone up a bunch in the last 2 years. If the one you're looking at is a real one, all the parts are correct and there is no rust and it's mechanically sound, I'd say it's a good deal, without being able to look at it myself of course.