Ideally, you have two people. since I am usually alone, I have made up a tool from 1/2 " water pipe with a sliding pointer on each end so I can do it myself. The pointers are about 10" high, and ground to a fine point on the tips, with a washer welded to the set screw so you need no wrench to adjust it. Simply lay it on the floor and set the pointers to the front line, and then move it to the back side and check the reading there. Also, not mentioned yet in this post, is this method is fine when setting up a new build with new king pins and bushings. In that case I believe it can all be done off the ground. However, on an older with perhaps loose bushings the car should be on the ground to see what is actually happening while going down the road with a load on it. Even a new build, I would check again after its on the ground to see if things have changed.