Use you gut instinct, but some clues are if the Numbers dont line up very good, but the downside to that is that even one that has never been touched/tampered with, will not line up perfectly, so talking with the guy, the shape that the cars in, how bad is the rock chips on the front bumper, look in the wheelwell and see how muddy/greesy the fender liner is, etc. and price, you have to add it all up if it works then buy it,, if you have the slightest feeling something isnt good then dont buy it. good-luck