It's not always cut & dry either. There was the remains of an old town in NH I used to poke around alot. It was a flood control area and under the jurisdiction of the Army Corps of Engineers. A large sign posted at the entrance had all the rules for camping, riding bicycles, etc. and included a specific paragraph that said metal detecting was allowed but that anything over 100 years old should be reported. Well one day after going there regularly for years I had a ranger in a Chevy Blazer with government plates pull up next to where I was standing. He procedes to tell me that I can't be there with a metal detector. So I politely mentioned the sign to him. He gets all puffed up and tells me that when HE is on duty HE makes the rules and that if I don't leave immediately he will arrest me and take my equipment.. A month or 2 prior I had a different ranger driving the same truck ask me if I was having any luck and pointed me to where the ferry used to cross the river and tell me that other guys had made some good finds in that area...

Here it is in case any of my NH friends want to take their chances.
Hill Village NH: Relocating An Entire Town of Hill Village New Hampshire