Evidently you can detect some, others you can't. The web site for the Chattahoochee Nat. For. in NE GA says it's okay to detect. Member ARC mentioned it's illegal in FL Nat. Forests. You better check before you go.
Do not become the latest victim of "No one cared UNTIL you asked" phenomenon. I am assuming this is a form of NFS ? If so: NFS does not disallow MD'ing. Just avoid obvious historic sensitive monuments.