Some fancy artists out there will use calipers to help them make adjustments to their work. You don’t get fancy around here, just useful and sometimes cheap. This trick of mine falls into both categories. I use those cheap and abundant clear rulers you can pick up anywhere to measure both my subject and my paintings when they get out of whack proportionally. That is to say if I am working at life size. Normally I have a really strong sense of proportion when I am drawing or painting, but I don’t always “nail it”. That’s when I pull out my trusty little ruler. It is such a useful tip that I keep one of them always nearby with my brushes while I am painting.
A note of caution: if you are painting from a model, they usually prefer for you to ask permission to measure them. Just saying.