Maybe you can find a dyno somewhere..... Usually it's measured in grams pr mm, that is, how many grams do it take to compress them a millimeter. HPI measure it, when the spring is compressed to half the unloaded length.
However, which brand are they? Maybe somebody do know them.... much easier....
Schumacher Mi5 - Sanwa MT-4 / RX-462 / KO Propo PDS-2123 - Nosram 5600 X-treme Saddle Pack - LRP SPX Zero/X12 17,5 - Nosram Stealth Touch Evolution - MAAS SPS 9250