Suggestions for making sure it's safe (without me unscrewing the bottom)?
Have you weighed it? According to the spec sheet I linked to above, the total weight of a loaded projectile is about 55 lb., and the weight of the explosive filling is just over 7 lb. So if your shell weighs about 48 lb., it's probably had the ammonium picrate (Explosive D or dunnite) removed, and if it still weighs about 55 lb., then you must assume that it's still fully live.
Honestly, I don't know what to tell you to do if this thing turns out to be live or potentially live. I'm sure the last thing you want is the local EOD or bomb squad camped outside your doorstep, and, moreover, I am unaware of what the legality of owning a live anti-aircraft shell might be. Might be the Navy considers it stolen property. I just don't know and would suggest you act very cautiously.