Place the spring on a level suface, arch up.
Measure the distance from surface. Lets call it Free Height.
Weigh yourself. This will be Weight.
Holding on to something, carefully load the spring with your full weight.
Remeasure from the surface to the same place on the spring pad as before. Call this Loaded Height.
Carefully get off the spring while holding on to something (or you may get tossed).

Now do the math.
Distance displaced = Free Height - Loaded Height
Spring Rate = Weight / Distance Displaced

Weight is in pounds force, Distance in inches, therefore Spring rate will be in lbs/in.
This is crude but approximately correct.

If you're a bit more ambitious, pull a string from eye center to eye center and measure the distance from the spring to the mounting pad. This is the free arch, or darn close to it.

After doing this, please share along with the exact part number of the springs. Other details you may wish to note is spring thickness (if you have a caliper or micrometer) number of full and partial leaves and bushing diameters. This will be good for your own reference but will also help others in the future.

Later you can do the same on your SS springs. In that case, you'll have to check both the left and right asthey may be different in free arch.