I usually approach them with some sugar coated truth,mamm n sir,mr n mrs them to death,Tell them that id like to detect/hunt on there property,Tell them that i`ll get there permission before taking any recovered objects of value from there property,etc. Also let them know in a round about way that you wont be digging the trenchs of vietnam across there land or leaving gopher holes behind. Just make them feel they can trust you not to vandalize or destroy there property. Private Property permission is a hit n miss kind of thing though,Some ppl could give a frick less if you came in with a two box and a bulldozer,Others will follow you around day and night making sure you dont dig up grand dads lost fortune. I avoid hunting private property as much as possible these days but i have recovered alot of old coins on private lands in the past.
HH and Good Luck,John