Papaw, this is a generally accepted answer, that most people wouldn't blink twice about: "Check with locales who've detected there a long time" or "check with a dealer or club there". As *certainly* those people who live and hunt in a certain geographic locale .... would ... doh ... know where they can hunt w/o problems, right?
But let me ask you this: if you did such a thing as you suggest (inquire ahead to long-time local hunters), and they told you "such & such parks are fair game" or "such & such beaches are fair game", then you (and even the most skittish of hunters) would interpret that exactly as it says. RIGHT? But you will find, if you go a step above them, and start looking this up "for yourself", that some of those places they tell is "ok", might, in fact, have verbage about "collecting" or "digging" (gasp, say it isn't so). Then how do those long-time locals simply "do it"? Easy, it's ... essentially .... "grandfathered in", and no one cares.
I'm the first to agree that .... if no one's having a problem, and if "everyone just detects there", then .... how can anyone argue with that? But what's interesting, is that this really does mean that "Actual Practice" is what dictates. NOT "actual law", in that case right? (or ...at least .... things that could be morphed to apply if enough people went asking for clarifications).
We have places here in CA that have simply been detected since the 1960s (early days of detecting), and it's not a problem (unless you're being a nuisance or something). Yet, oddly, if you looked long enough and hard enough, you might find things you think preclude you. Like if you were an outsider getting ready to move here, or vacation here, so you "looked up the rules" and found things about "removing" or "digging").
Thus, I don't put a lot of stock in actual rules (unless it truly said "no metal detectors"), and instead will go by what others "just do".