I usually just eyeball them. If you find that you want to double check the setting you can measure the distance from the float arm to the bowl gasket seat (on the carb body) Measure it back at the float arm pivot point then take another measurement at the far end of the arm and compare the two values. The reason I like to eyeball the arm is to make sure it is not bent in any way. I can see any deformity by eyeballing it as I slowly rotate the carb in my hands. It probably has something to do with the 20+ years of digging into carbs.