..... but just ask the police to be sure. after all, if you don't know, how are we to know? .....
Gleaner, no, a person does not need to ask the police, (or parks dept, or city-hall clerk, or school admin, or ..... whomever), "to be sure". There is another way to know:
Look it up for oneself.
City muni codes, laws, rules, regulations, ordinances, are all public knowledge, in print, somewhere, somehow. If the city does not have a website with those things listed, then at least on the front desk at city hall, for public viewing. Because legally, it has to be available to view, somewhere. Some bigger cities are now subscribing to this service, for instance:
State Listing - Municipal Code Corporation
Once you look it up for yourself, and see nothing that says "no metal detecting", well then gee, I guess that answers the question
You may ask: "well that's time consuming, and my city (or county or whatever) doesn't have it out in binder form at the front desk, etc..., so WHY NOT just ask a cop, or gardener, or clerk?" Here's the answer: Because all too often, even if you phrase the question carefully to say "Is there any rules against metal detectors in the park?" (thinking that would put the burden of proof on them to PRODUCE or CITE such an actual specific rule), yet it can backfire. There are examples I've heard where they simply say "no you can't" (as if you'd just asked their permission, and not asked if there is a "rule"). And if you object and say "but where is that written?", guess who's going to win that debate? They can just morph something else to apply to your question (the dreaded alteration, defacing, and digging type stuff), and do you really think you're going to win that debate? Mind you, that very person you're asking, perhaps NEVER would have given the matter a moment's thought (till you asked).
That's not to say that sometimes the carefully worded question does go in the md'rs favor, and they might say "no, I don't see anything addressing the subject of metal detectors". I'm not saying that might not happen too. But for me, I wouldn't take that chance, and will look it up for myself.