Wrong forum, but laws depend on the state, and sometimes even the city / county. Your question is really general, but you will have a tough time getting permission to detect government owned properties (at any level of government). Those who ask are usually told no whether it is actually legal to detect on the property or not (the government worker doesn't want to say yes because they are unaware of a specific law saying metal detecting is OK). Unfortunately, you would need to do the research to find the legal right to detect, and in most cases that law doesn't specifically exist (granting the right, that is)... so then it falls into the gray area of there being no specific law against it, but not knowing if that means it is ok or not (because it would fall under some other law regarding destruction of government property, etc.).
In short: if you ask, you will (more than likely) get told no. If you detect without asking, you MIGHT get thrown off the property (or worse, but usually not worse) and/or your finds confiscated.