Conqueror posted this at the Tinker's Guild Forum a while back and I found it very informative.

The spring constant K is generally measured in Newtons per meter. Simply get a weight of known mass in grams, divide by 1000 to get mass in kg, then multiply by 9.8 to get newtons. Use something like a 50 newton or 100 newton weight, put it on top of the spring, and see how far it gets compressed. (Newtons)/(distance compressed, in meters) gives you K. Bigger K means stiffer spring, and vice-versa.

CQ