how does a wheel alignment machine measure caster?
 

How does a wheel alignment machine check caster?

There's some measuring device clamped to the rim that has a gravity reference but nothing I can see to reference the steering axis.

Thanks,
Sixto
93 300SD

It measures the difference in camber with the wheel turned 20 deg in and 20 deg out. It has been years since I did an alignment but we used the old fashion method of a float guage on the spindle. Worked for cop cars and ambulances so it had to be good.