As the others have said, it depends on who owns it. They can be federal, state, county, city, or private. And just because it's federal or state still doesn't necessarily mean you can't hunt it. You can hunt fed or state land, as long as it's for modern stuff (and that's all you've found, right? 8)) or things you yourself lost (like that boyscout ring you lost there when you were a kid, right?)
If it's an obvious historic landmark (Bodie, etc...) then assume you can't. Odds are, if you're touring the easy-to-find, well-known ghostowns, you're going to be dissapointed anyhow. Anything that's easily looked up in coffee-table type "Sunset Homes and Gardens book of Ghostowns" are usually just tourist traps, or hunted to death, or junky 4-wheeler partyer enclave cesspools of junk. A friend and I travelled the southwest with such "tourist" ghost-town books in tow, and soon realized that those are the places to AVOID. Better to hit the sites that are off-the-beaten path, with little to no record of them. JMHO