fun-blonde, even though I had chimed in that a person can look up the laws/rules for himself, one thing I didn't add in to that answer (to keep things short), is the following preface:
"If you are skittish, you can look up the rules/laws for yourself...."
So to be honest with you, when I'm travelling around, ....... no, I don't study each entity's website, or go to city hall and leaf through binders full of minutia, etc.... I just go detecting. I mean, barring "obvious historic monuments", then no, I do not bother myself with doing that "look up the rules" step. I figure if someone has an issue, they can come let me know. Because odds are, if there IS a rule, that an average person could not have been expected to know, then ...... I believe you will be alerted/warned first. I'm talking innocuous normal parks, beaches, forests, etc... Not historic monuments, where...... yes .... you would be expected to know, even if not plainly posted on a sign.
So my advice to look it up for oneself, was only for those who feel skittish. Not a mandate that is necessary, that every md'r does. Unless they had some real reason to suspect, or worry, etc... at an individual site.