Actually, yeah, kinda. More typically, it's a tape measure around the whole muscle group, but yeah, when it comes to certain back muscle groups, yeah, they actually do use calipers or other measuring devices and then structure future workouts to get the body as symmetrical as possible.
One of the things bodybuilders will tell you is that three things go into making a bodybuilding champion: genetics, hard work and supplementation (up to and including HGH, roids, insulin (old days), myostatin blockers, etc)
What genetics does for a bodybuilder is to define what the shape of a muscle will be once it is built up. NO amount of supplementation can change that and hard work can only go so far.
For example, some current bodybuilders have Steinerism or a "chest gap" See this image of Terry Crews
Now look at this pic of Arnold
Arnold's chest is MASSIVE and yet his entire sternum is covered. Part of that was a different workout regimen. Arnold was a power lifter first and switched to bodybuilding, so he focused on heavy weight for mass and regulated his workouts to include rest. He was very mindful not to overwork the muscles which allowed him to fully augment his musculature while avoiding modern issues like Steinerism, belly bloat and other issues common today. Now, whether it's caused by insulin use, overwork or injury (all things which could deteriorate the medial pectoral nerve), it's an issue. The wrestler, Steiner, apparently also had a serious lung injury which required surgeons to crack his chest open and it looks like he didn't let the nerves get deep enough into the muscle before resuming lifting. Had he waited a year to resume lifting, it might not have been that exaggerated.
Some people naturally have a degree of it just based on where the muscle attaches to the sternum.
Also, look at anyone's face. It's asymmetrical. You can do a simply photoshop test to see this. There are videos on youtube where they show what a person would look like if they were symmetrical and some of the pics are startling.
The body's the same way. Not everyone has the same development on each side. A leg might be longer, an arm, etc which affects how it will respond to working out (torque equals force x perpendicular distance) which is why equally strong people can handle different loads depending on arm length. We see this with long arm OL vs short arm OL who do substantially better on the bench press. Sometimes, guys with a longer arm will have to do extra work on the longer arm to get the muscles to look symmetrical even though technically it means he's on purpose giving himself a strength imbalance.
It is a remarkably rare person who's body is as close to symmetrical as Arnold's is.
And yes, bodybuilders do tons of measurements to ensure symmetry as it's one of the categories for judging and a lack of symmetry is basically a fail, it's worth that much.
Even looking at his back, his development was remarkably similar. The slight differentiation is more a factor of where the light is coming from and the fact that he's facing his head to the right and his right elbow is slightly lower than the left. It's not perfect, but remarkably close. Note the slight lack of symmetry in his abdominal muscles. There's a slight raised offset on the left side.
Sorry, that's a long way to say, yeah, they do all of that and more.