I have a 1982 el camino that is not currently being driven. A few weeks ago I went out to move it and it wouldn't go in gear. Transmission fluid was a little low but not much. I added some fluid. I haven't done much since. What are the best steps for troubleshooting this problem?

Hmmm... not driven means no fluid has been keeping critical seals and valves lubricated and functional.
You could perform a line pressure test to see if anything is moving but your best bet would more likely be in the form of a replacement with a known-good used trans or a remanufactured trans from a reputable source.
Replacing the trans is a big deal and you won't want to spend time and money trying to salvage a dead one nor installing a replacement only to have it fail shortly after install.