If you do resort to calling (as mackaydon suggests), be sure to phrase the question in terms of whether or not there is anything in the rules that would restrict metal detectors. In that way, the burden is on them to show you an actual rule, actual wording, etc.... Do not phrase it like "can I metal detect?" or any other such way that sounds like you're asking their permission. Because if you do it that way, you might merely get someone to tell you "no", just because they said so (because perhaps some desk-bound clerk thinks you're a geek with a shovel).
Personally, I just go, unless posted otherwise (and of course, stay clear of obvious historical monuments). But if you are skittish, better to look it up yourself (lest you get someone to morph something stupid, like "don't disturb the vegetation" to apply to your question). Most park's departments have websites (do a key word search under "metal detecting", for instance), or the code books are usually out on the city hall desk for public viewing. If it is silent on the issue, then just go.
