I'm constantly grabbing my trusty proportional calipers and double checking my measurements. While work like this figure is a figment of my imagination and I have no actual model standing there to reference, I do refer to anatomy books, charts and a life-sized skeleton that I keep in my studio.
Additionally I will sometimes take measurement of myself as a general guide - if my sculpture is an adult female, that is. Naturally that won't work if I'm sculpting a man.
Here I am taking a measurement of my forearm near my wrist using the larger end of the calipers. I'm working roughly at 1/4 life size (this sculpt is about 16" tall - 16" x 4 = 64" tall, or about 5'4" tall. I'm about 5'5" so I'm a good fit for measuring this particular sculpt). I have the the calipers set to 1/4 scale.
Now I simply flip the calipers around and use the smaller side to measure my sculpture's wrist. I do this often as I work to be sure the proportions are working.