Mackaydon, I agree, steer clear of obvious historic monuments, open archie digs, etc.. Same is true for the USA, or anywhere. But as far as inquiring about other areas, one needs to be careful that he doesn't fall prey to the pyschology I alluded to above. Ie.: you can sometimes get a no, from any perceived authority you ask, simply because of the way they couch their answer, or the way you asked your question, etc.. When in fact, no one may actually care, and there may be no rules (or at least not rules which, in practice, really apply).
There have been scores of examples of "people inquiring" before heading to Mexico, and getting a "no". Perhaps they ask a border consulate, a lawyer, the hotel, their cruize-line, etc.... Sounds logical, right? I mean, who better to ask, than Mexico themselves, right? But the psychology works like this: the mere fact that you felt you must ask, merely pre-assumes something is wrong, to begin with. Ie.: would you have asked if it was ok to fly frisbees? So with this inference in mind, the person you ask will try their durndest (you can never be too safe afterall, right?) to morph your question to fit with something meant for federal parks, or historic antiquities, or disturbing sandcrabs, etc.... And imagine those person's surprise, when they leave their detectors at home, only to see others hunting the beach resort with no problems.
If a person really feels they must inquire ahead of time, simply talk to hobbyists or dealers in that area, and see if and where they detect.