Essentially you just need to find out how much weight it takes to compress the spring 1".
Get something with a known weight, then use something to measure the height of the spring like a string. When the spring compresses 1" then that is the weight per inch.
You could put the spring in a shop press which has a gauge. I'd be careful and just collapse it one inch which is all you really need to do.
Performance one step at a time.