I think the best way to learn that if something is
PMD on a coin is to go out and damage some coins for yourself. I know I damaged quite a few when I was a kid. Hit them with hammers, with rocks, with a BB gun. Dip them in acid. Take a file or a dremel tool to them. Stack up three or four and squeeze them in a vise. See what happens! Take a close look at coins you pick up out of parking lots to see what damage being ground against asphalt for a week does.
Then, learn how the actual minting process works, and what can and can not happen at the mint. Once you know how genuine errors and varieties can occur, it's a piece of cake to tell what is just damage.
Dave