Hopefully this will help some of you guys out a bit. Most local police are NOT going to know the laws about metal detecting, unless they are in a specific area which has a large number of people going there to do it, such as Manassas. If you want to get an answer for a certain jurisdiction, call the Magistrates Office. These are the guys that will determine if there is probable cause for an arrest. Get the name of the person you spoke with. If you should happen to get stopped by the police for metal detecting, which is fairly unlikely, Tell them that you checked with the Magistrates Office prior.
There may be park regulations that prohibit metal detecting. If that is the case, they can tell you to stop, they can ask you to leave, but you can't get into legal trouble for it, unless you refuse to stop, or leave when asked. If you refuse to leave, then you are trespassing, and you CAN be arrested for that. So far, I've never been asked to stop, or leave. I don't hunt in the common areas though, and am usually well away from the other people using the park.
I've hunted in several parks in Northern Virginia, and never had an issue arise from it, including the ones which were Civil War forts. Unfortunately they are so full of garbage that hunting them has been absolutely useless. I've had much better success hunting private land, and turned up a fair number of Civil War relics, and just the other day my oldest silver so far (1820 dime).
Upton Hill is NOT a state park. It is operated by the Northern Virginia Regional Park Authority. I went through there in the late 70's, right when they were opening the swimming pool and mini-golf areas and found a couple of three-ringers and one barrel band, but that was it for relics. The land that is now the park was for many years the headquarters of the American Nazi party, and the Nazi's left a lot of trash behind.
It depends on your hunting preference I guess. My goals are Civil War relics. If you are a coin/jewelry person, the parks may treat you great. For me, they have been a waste of time.
Good luck all !