Ask and you shall receive - or not !
It's really not as hard as one may think. Just be nice, tell them who you are, where you live, and that your hobby is metal detecting. Tell them that you'd love to look around their property, if they don't object. I also find that it's a good idea, to have some appropriate calling cards printed up, with a nice MDing logo, your name, address, phone, etc., and assure them that you won't leave a mess, but should they find something out of order, or to their disliking, to please feel free to give you a call, and here's how to get in touch. Also tell them that when you come back to hunt, you may have a friend with you, if this should be the case.
I always throw in, "and if we find that treasure chest full of gold, we'll definitely split it".
Still looking for that chest, though !
I've got one old (early 1800's) place, which is now a B&B. When I returned the third time, the owner actually came out and met me in the yard, just to say that I need not worry about coming to the door, or calling ahead, in the future, but to just feel free to make myself at home, any time. Even offered me ice & water in the house, should I need to quench my thirst. Now, I feel very comfortable dropping in at any time, whether anyone is at home, or not. I've gained their trust, so it's an open door from here on ! That's the kind of relationships you need to build, and be careful who you take with you...........