you can use a long straight edge, hold it against the RR rim, then measure the distance front and rear on the RF rim
I had a tool made that is 50 inches long and has a round peg on one end that was machined to go inside the rear axle, and for the front we had plates made that bolt to the hub with the tire still in place, RF is the same sixe as the RF tire, LF same size as the LF tire for the driver to be in place in the kart on the scale stand