SWR, what you're talking about, is a different situation: That of a rule existing, but that isn't posted necessarily on the wooden sign at the park entrance. If someone is skittish of that scenario, they I STILL say they shouldn't ask. Instead, they can look it up themselves. Most cities have a website with their rules and codes posted there (do a key-word search under "metal detecting" or whatever).
The problem with "asking", is that you can risk a "no", simply because someone felt like it, EVEN in the absence of any actual rule. And this can hold true even if you carefully phrase it as: "Are there any rules prohibiting metal detecting?" (thinking that this puts the burden of proof on them to show such a written rule). Because the clerk could say something like "no, we would prefer you not do that". If you challenge them and say "but where is that written?" they can merely say "we don't want people tearing up the park". And then you simply get into a p*ssing match of whether or not you only dig surface clad, or whether you probe only, or whether or not you'll leave no trace, blah blah. Who do you thinks going to win that debate? And truth be told, no one probably would have ever paid you mind, if you had just gone. I mean, it would be like if I asked them "can I spread peanut butter on my shoes and march around the park with a tin-foil hat on?" They might say "you need a parade permit etc...." But if I just did it, do you really think anyone cared?
JB7487: Congratz on your "permission" to metal detect in two parks. I gaurantee you that I can JUST as quickly go to that SAME clerk, and get your "permission" just as quickly revoked. You see, permission simply means the person granting it simply didn't think through all the facts. All I need to do is mention things like "digging", "holes", ARPA, cultural heritage, city artificats ending up on your mantle, non-returned personal jewelry, etc.... And guess what will happen to your "permission"? And if you think your permission slip will do you a bit of good when some busy-body comes up to gripe, guess again: You can still get booted. There's been many posts of persons who are quickly over-ridden by an irate cop or gardener, who sees things differently. And if you push your little permission slip authority, they merely call down to city hall, and say "the guy is making a mess, who the heck 'permitted' this??"
Lastly, you say "the worst that can happen is that they will say no" Well GEE, that seems like a pretty dire acknowledge outcome to me! Why would I want to basically "get a rule written", where none previously existed before, and where no one really cared before (till you asked)? I mean, what sense does that make? This happened in my own city: Someone newbie who moved to our city takes it upon himself to go down to city hall and ask "Is metal detecting allowed in Central Park?" (afterall, "better safe than sorry, he probably reasoned). Some desk-clerk tells him "no". So when he attends our local detecting club meeting, and sees others who have entered in some old coins "found at Central Park", he raises his hands and objects "I thought detecting was illegal in Central Park?" The rest of us turn around and say to him "since when? who told you that?" Heck, we'd been detecting there for 10+ yrs. by then, and no one had ever had a problem. Well you can guess what happens next: half the room of hobbyists think "oh no, the parks are off-limits now", and the other half says "nonsense, no one's ever said anything to me, you probably just got some desk-clerk who gave the easy answer, and I'm not going to let it stop me". That was ~20 yrs. ago. And to this day, you can still go detecting in the parks here, as long as you're not being a nuisance, sticking out, or leaving holes. So you can see that the person who thinks he needs to ask (where no rule really exists), merely precludes himself from a good hunt site. What sense does that make?
