How to measure wheel offset?

[Brian794x4]

Hey guys, i posted before about finding some 15x10 ford steel wheels at a wrecker. I want to see what kind of offset they have so i can see if they'll work for my truck with the lift. Is there a method with a tape and strait edge or anything? And on a side note, my 79 150 4x4 is lifted 6" and ill have a body lift... how much of offset will i most likely need? Ill be putting 38x15 wide tires on it in the spring, and probably flares i think too, if that helps. Thanks alot for any help! Brian

[SoCalDesertRider]

lay the wheel down face-down and put the straight edge accross the wheel lip and measure the depth to the inside of the wheel mounting surface, where the lug holes are.