I use a white board and tally each pint. For me, an empty keg weighs about 8 pounds. I weigh each one after I fill it and subtract 8...the number I'm left with roughly equals the number of pints in the keg. It's not exact, but it's better than flying totally blind.That's not a bad idea at all.. Weigh the keg empty (note down weight) and weigh it full (note down weight). Then you can always compare and see how much is left... Though it would require pulling the keg out of the kegerator to do so.
For people with keezers you could gently pick up the keg with one of these
American Weigh Scale American Weigh H-110 Digital Hanging Scale, 110 X 0.05-Pounds
If you know the tare weight if your keg you can use Beersmith Mobile's weight to volume calculator to derive the remaining beer volume.
I don't even think you would need a calculator. A gallon of water weighs 8 pounds and there are 8 pints in a gallon, therefore, a pint weighs a pound. I doubt there's any weight difference between beer and water. Subtracting the tare weight of your keg should give you the number of pints that are in the keg. If you want the total oz. just multiply by 16. I just use the pint as a unit of measurement because all the numbers fit perfectly and most beer glasses are pint glasses. Usually I get 1-3 pints more than my measurements would indicate, but I've always attributed that to pouring 13 oz of actual beer and the rest is head.