You will know when you see it. Is the paint cracked, are the bottom edges pealing, is the bottom rusted out. Does it mount up flush? It's a challenge removing these things. I bent the shit out of mine when I removed it. Although I wasn't planning on reusing it. Plan on replacing it again sooner or later.
I wouldn't pay much for it unless it was a 8 or 9 out of 10 in condition.