Just to add.
You can monitor the fill level by weight, or if the beer in the source keg is cold, condensation on the receiver keg (at least that works in humid MI).
If there's not enough humidity to form much condensation, you can also get a pretty good sense of the beer level in the receiving keg using your fingertips, assuming the beer being transferred is cold. Just leave your fingers in contact with the keg for several seconds - you can easily feel the difference between the fingers that lie under the liquid level (they will be cold, and _stay_ cold) and the fingers above (which will not feel all that cold for long, as your fingers will heat up the metal a bit).
My wife says it looks like I'm doing a Vulcan mind-meld with the keg when I do this. I'm ok with that.