? on doors nos or not

[bark]

Hi, Was just wondering if you can tell the difference from GM and aftermarket parts. I just picked up a set of doors for my 77, the guy bought them from GM a bunch of years ago, but never used them, and I was wondering if GM stamps there doors some where? not that I'm to worried about it, they seem very well built, nice and thick, and look to be dead straight. I also got some fenders and cab corners from lmc and they seem like very nice parts.