Idea, trying to convey in words.
Place spring over a hole in a board. Put long screw with fender washer through the spring and hole. Hang weights off the screw (don't forget to account for the weight of the screw/washer) and measure the deflection of the fender washer relative to the board.
Spring rate, k = weight/deflection
Team Associated | Reedy | Airtronics
RC Excitement: Fitchburg, MA | RC Madness: Enfield, CT | Horsham RC