OK, I spoke with a friend who was involved in the arsenal rebuilds circa 1966, and here's what he had to say about the topic of finishes and rebuilds:
So there's no simple answer that will fit all rifles. It'd have to take an inspection of the rifle by someone "in the know" to determine whether or not a given rifle was a rebuild, and even then, there will be different shades of certainty in the answer. That having been said, if you got a WW2 M1 straight from the CMP that was billed as a Service Grade, and the finish was uniform across all the metal, it would probably be safe to assume you've gotten an arsenal rebuild, especially if the barrel was post-war dated. Now if it's a post-war M1, determining whether or not it's original or a rebuild gets fuzzy.
You're making my head hurt.
Edit: Anybody have any Navy Garands in their collection?