you could weigh the keg empty on your bathroom scale and weigh it during your drinking. Also, weigh a single or set number of glasses of beer. Then knowing the weight per glass of beer, divide the weight of the keg plus beer minus the empty keg weight by the weight per glass of beer. then you will know how many glasses are left.
--------------------- = #of glasses
weight per glass(es)
kinda geeky but my first thought.
second thought: take keg out of fridge watch for condensation to build if your humidity is high enough. the condensation should stop at the line where the beer is highest.