Yes, the "magic of the internet" and a printer from the office supply store. The trouble is though:
A lot of smaller cities and counties don't necessarily have their "municipal codes" (laws, charter, and such) posted on-line. Yes, some do. You merely click on the parks dept. And from there, often a sub-menu has "Park rules", and from there, you determine there is no rule that says "no metal detecting". You print those pages, and presto, problem solved. If some busy-body approaches you, you show them the rules. Or if you have a doubt or suspicion that perhaps even though the PARK'S codes dont' say thing anything about that, perhaps the city-wide rules have something (very rare and un-likely). Ok, then you click the 'print' button on the entire muni-laws section. Presto, problem solved.
But again, another problem you can face, is even though none of those "rules/laws" listing might not have any prohibitions on md'ing, yet .... let's face it .... someone can "morph" other silly stuff to apply. You know, the usual "defacement and alterations" verbage. To which I would say those only apply to the end result. So if you leave no trace, technically you haven't broken those. However, someone can level the charge, regardless. Or stuff about "collecting and harvesting" or "cultural heritage" or "disturbing earthworms", etc.....
But I do not assume, from the git-go, that any of the above things apply (until "told otherwise"). Because to assume they apply, is that you might as well give it up now, and stick to private property. Thus I will only figure that a *specific* prohibition is the only thing that applies, till told otherwise. And you're right: You print out a list of the rules, and ..... there's your "permission" slip

If someone has an issue, they're welcome to tell you. Avoid such encounters and avoid such lookie-lous to begin with. Hunt at low traffic times.