Stamped vs milled is more of an AK receiver issue - ARs, it's whether the receiver blank was cast or forged before machining started. Forged is tougher than cast, but there isn't a lot of stress on the lower of an AR. If I had two identical condition prebans in front of me, one cast and one forged, I'd pick the forged one, but otherwise I'd buy on overall condition first. Olympic has a page up on
identifying cast or forged that's worth looking at.
Oval holes instead of round, cracks around the buffer tube or pivot pins, or serious dent's & gouges are the sort of thing to look for, but all are uncommon. If it's a full rifle, you can at least do a function check to make sure all the controls are working properly. Make sure a mag will seat, and no one parked a truck on the mag well. Finish condition will be an indication of how the rifle was treated, but the receiver can be refinished if you pick one up that's tired looking. If there's signs of amateur gunsmithing being done on the lower, I'd walk away fast.
Good luck on the hunt - I bought a preban cast EA a while back, which had bad finish and damage to the hole for the front pivot plunger. Repaired it, runs well, just not pretty (yet). I still look for others, of course.
You can look around online and get a feel for which rifles were decent quality back then. There were some manufacturers that were rather inconsistent. Keep in mind that you're looking for defects that can be traced to bad receivers - if the company in question had a rep for using crappy BCGs, that's not the lowers problem.