you can build a tester with a plank, a hook and a weight.
You need marked springs to calibrate it.
The weight pulls on the hooked spring, you draw a mark where it stops going down with gravity.Any other spring with the same weight attached will give you an idea of the hardness difference by descending lower or higher than the reference mark.
it is quite precise because you get an answer in spring stiffness, not fps.
Differently marked springs with the same stiffness will be visible right away.
like a spring balance to get their value in newtons:
