If I were to check a spring I would measure its extended length and then measure the force per change in length to design compression. I would look at that data to see that it makes a smooth line and compare it to the manufacturer's data if available.
How do you check a spring?
Oh, my H&K dated KI is in my pants at this moment. Its spring has only ever been momentarily expanded.
If you're learning or taking advise on some tidbit of fundamental physics/technology, do not take it from a purveyor of the tidbit, of course they are too glad to sell to a sucker - there's one born every minute. "This way to the egress, she walks, she talks, she crawls on her belly." (P. T. Barnum)