I think that there are tools to measure the amount of force it takes to compress a spring a given distance and to compare it to factory specs. My Auto Service Repair textbook only says:
"Be sure to mark spring top and bottom (if different). Also label which side of the vehicle it was removed from. If spring requires replacement (sagging, bent, broken), carefully relieve compressing tool tension. Remove spring and discard it."
Here's what the factory manual says
