Actually Rob that is not true. If you can measure CAMBER then you can measure/ calculate caster. My old Ammco caster/ camber gauge measures camber, and caster is computed from that by what amounts to a rotary slide rule on the instrument.
You measure camber with the wheels turned one way, 20* in, then measure camber again with wheels turned 20* out. You figure the DIFFERENCE in that reading and multiply X 1.5 That answer is your caster