I used a kitchen digital weighing scale. Find a spring that you can identify and know what the spring weight should be.Put the spring on the scale and compress it until the weight matches the published weight (this should be an inch btw) and measure how far you compressed the spring (this is a bit tricky). Now get an unknown spring, compress it the same distance (ie an inch) and read off the weight on the scale - sorted.
There is a proper tool for doing this but it is quite pricey.
https://www.ebay.co.uk/itm/G-FORCE-G....c100010.m2109