We often grind our own chuck, it is so much better than the store ground option.
We eyeball the amount of fat.
Have never had a piece of chuck so marbled that we did not need to add some of the white firm fat.
Saw one of those things the inspectors used years ago; at that time, as I remember, they just weighed the meat patty, cooked it to death, and weighed it again. But that was a long time (40 years or so) ago and things may be quite different now.
I find I have a tendency to underestimate the amount of fat, so I always add a bit more than I think is necessary.
When I used to work in grocery stores, again many years ago, they would just eyeball it also. Never saw anyone test anything. How they do it today I have no idea.
When making hamburger or sausage, we always fry up a small test piece.
If it needs more fat can always run it through the grinder again.