Interstate highway mile markers are a pretty reliable way to judge the offset. Sometimes state routes (2 lane black top) will also have them. The thing I did many moons ago: grab a car I knew read correctly, drive it about 10 miles, turn around. Note what it read out / back / combined. Take the exact same run in the one I knew didn't (tire, axle change) then calculate the offset. Put the tail on ramps to keep the fluid in the trans and swap gears then re-run the loop. As a final sanity check, I go looking for one of those road side construction zone things that flash your speed at you.