you can measure the springs.. but if the springs are older, weight inside jeep, and other variables they could sag or screw your measurement. the final ride height and rate of a coil spring are affected by the wire diameter, the coil diameter, the number of turns (coils) in the spring, and the free length. heres a few I found.. take it with a grain of salt though,
Spring Coil Rates:
Tera 4" TJ coils give 4.5" of lift on an XJ and have a 200 lb/in rate , 20.25" tall unloaded
Tera 3" TJ coils give 3.5" of lift on an XJ and have a 170 lb/in rate
Rusty's 4.5" coils give 5" of lift and have a 150 lb/in rate, 22" tall unloaded
RE's 4.5" ZJ coils give 6"-6.25" of lift on an XJ and have a 200 lb/in rate, 22" tall unloaded
Rustys 3" coils give 3" and have 150 lb/in rate.
Rancho 3" coils, at least the old coils I had give about 2 1/4" of lift on an XJ and have a very high 235 lb/in rate.
OEM is supposedly 147 lb/in rate.
OME coils give 2.25" - 2.5" of lift and have a 148 lb/in rate