Generally there is a stamping on the door somewhere if it is a GM door, but it will only say GM (from the ones i have seen) I haven't ever seen one in the same place, and some I know were original, I never found it on. The aftermarket ones are about the same. As for fit, the factory ones will have to be tweeked to fit, UNLESS it came on the truck you are working on, and even then the fit won't be perfect.