Two other non-contact methods I've seen are direct mass measurement (probably ebay the scales, it'll be like $500 new) or the most innovative thing I've seen is flow rate meters watching the CO2 source. See you can measure flow rate of the beer but that can get weird with bubble formation and sterilization but CO2 is clean and biologically boring and single phase. You can buy a flow rate meter with digital output for immense expense but its easier to use an analog one with a webcam. Big fun!
Measuring the co2 flow rate is an idea I hadn't considered! I only want to know when beer is being poured and not actually how much has been poured so measuring co2 may be a bit simpler for cleaning and actually wiring up too (although the wiring issue is specific to my setup)