I've done this, I think, on one occasion. I've tried it a couple times and only one time did it work out to the point where I said... yeah this is better, I'm going with it.
I use guns so that may account for my ability to do so (more direct sound, less reflected).
What I listen for is an imbalance. Does it sound like the music is leaning to one side or the other?
If so I compare wave forms. What I'm looking for is the peaks and zero-crossings to match up. On most occasions they do, and no adjustments can be made.
On this rare occasion that I speak of, they did not match up, and the adjustment ended up centering the music much better.