Is there a way to tell the difference between GM and repo sheetmetal?

[orange72blazer]

I'm looking at buying a project 1985 K20 that has a new doors, fenders, inner fenders, hood, tailgate, rad. support, ect. All of the parts still have the GM stickers on them except for the doors & fenders which have already been primed. I'm wondering if there is a way to tell if the doors & fenders are actual GM parts or not. I know most aftermarket sheetmetal is stamped someware on the part, I looked these over and didn't find any stampings.