How to tell the drop

[Jmann8686]

Bought a 72 C20 a few weeks back. It has been lowered. and I like the stance. I have not looked to much into it as of right now. But i was wondering if there was anyway to tell how much drop as been done to it as it sits right now. The frame has not been notched and there looks to be only about 1-1/2" to maybe 2" of travel before the axle hits the bump stop. Is there anything I can measure or should I look for part numbers. New to the lowered 2wd truck world.

Thanks