Usually a set of graduated turntables are used to measure caster, then from the numbers read off the camber gauge, the castor is calculated. I found that too slow and tedious so I have made a jig that fits over the top and bottom ball joint nuts, so now it's a direct reading, but there is no caster on the rear of these cars.
I did the measurements with strings as you do Cooperman for many many years, very accurate method if you take a little time and care. I made up the lazer as now I can see it from under the car as I adjust, so now there's no need to turn a bit, measure, turn a bit more, measure,,, just adjust while looking, lock off and lets go! Kinda like reading of one of them modern digital verniers - in a way. It's fairly simple and most of the bits I bought at the hardware (mostly square section aluminium), once it was done, I calibrated it on the bench with a 1 metre ruler.