For my field work, I used to have shoes that were exactly 12" long, and put painters tape on my pant legs at 9", 15", 27", 29", and 34" AFF; and 48" on my sleeve.
It was not accurate, but I would only need to measure if it looked like it was gonna be close.
After you do enough field evaluations, at some point you brain can eyeball it really close, probably within 1/4" accuracy - - much like how upright bass players know exactly where to put their fingers to hit the right note.
That said, I always rely on a yardstick or tape measure for final verification when measurement looks close.