The old/old way on a TC was with a ride height gauge that had droop steps on the back. I'm sure guys still use those. 1/5mm is common resolution now so a purposed droop gauge was the tool for a long time measuring an arbitrary point on the suspension.

Nowadays most drivers are using a hub mounted reference dongle. It's a little better tasked to measure uptravel and makes the droop/uptravel numbers somewhat applicable between all cars (it is not of course).

Mind you if you are old like me in the old old old days there wasn't a droop limiter. You played with your shock lengths and if you wanted to limit uptravel (didn't really need to back in the day) you used orings. So the process and the method didn't lend itself to 1/5mm measurement, although I'm sure there were guys that tuned to 1/2mm even way back.