How can i know fore sure if head has been milled

I just saw an interesting youtube video where the guy (SoCal homeboy) levels an aluminum head to 1 mil using sandpaper on a glass backing. Not so easy on cast iron. Anyway, he points to the depressed spots on head with a rough casting surface and says as long as you don't remove past that, you haven't over-milled the surface. I always wondered what those spots were for. I can't recall if I saw them on an old Mopar cast-iron head. Somebody here may know.