I just spent over an hour transcribing eight measures of a Joe Pass solo. I was able to find a few chord voicings right away, but I had to puzzle out most of them. If I couldn't figure out a voicing right away, I would listen for the highest and lowest note of each chord (the two easiest notes to hear). This gave me a reference point, and then I could begin to fill in the chord from there. I would listen for a minor, major, or dominant sound, and then I would experiment with different voicings of the same chord until I found a match.
A quote from a Joe Pass DVD has helped immensely. In the introduction (I don't remember which DVD), he said, "What I play is easy." He wasn't being facetious. Whenever I watch a video of Joe Pass performing, he almost always grabs easy chord forms. I used the "this is easy" concept as a guideline to find the simplest way to grab each chord.
As I transcribe this solo, I really am struck by the simplicity of what Joe Pass played. He took basic guitar voicings and put them together in brilliant ways. I can already tell that I'll be reaping tremendous benefits from transcribing this and other Joe Pass solos. As I puzzle out each new voicing, I am either picking up new chord voicings or learning new ways to use voicings I already know. Eventually, this will become part of my own vocabulary as I explore unaccompanied improvisation.