To answer your question directly, the method most use is to measure between the center of the hub and the bottom of the fender flare (this takes tire size out of the equation). This info is all over the forum, so you can search for details and nuance. Even with these measurements, it still probably wont do much to help you determine which springs you have as truck/accessory weight, loads, age, all affect lift height. No two trucks are the same. OME springs come with a tag/label--if it's still there, that should give you the spring code. Mine was still barely legible after unknown but many years/miles.
And to the comment towards baldilocks, he is being helpful. Getting yourself a copy of the FSM will answer even more questions than this forum will, and is the proper first step for any new-to-you vehicle that you intend to tinker with.