Caster is the angle of the kingpin off vertical, and it's almost impossible to measure without some dedicated equipment, especially on a vehicle with a straight axle. Your carpenter's level will give you a ballpark guess at camber, not caster, but you'll need to fabricate a couple of extenders so you can hold it against the bead area of the rim at the top and bottom without touching the end of the spindle. It would take some pretty good trigonometry to convert the tilt of the bubble to a degree reading, although I've aligned a lot of race car front ends in the pits between races with a carpenter's level, a plumb bob, and a tape measure.
Jerry
