by using a flat piece of metal or plastic placed against the hub of the wheel and measuring the distance from the flat piece to the spoke. That is the back spacing on the wheel itself. Do the same for the hub of the wheel disc to the calipers. Might use a metal ruler square to make measuring easier on the wheel hub on the car. You need to exceed the measurement on the wheel hub of the car to clear. My last set of wheels cleared by 3mm or 4 mm.