I'm used to garbage workers opening the container before it's tipped, to keep the lid out of the way of the falling rubbish... also I'm used to seeing that kind of containers being cleaned on a somewhat regular basis (just by hosing them down). So whatever sensor you use it'd have to be really robust!
That said, the ones at the waste collection point across the street from me are usually kept open anyway. Just as easy. Waste is there for no longer than a day (except Chinese New Year when garbage collection stops for a day) so it doesn't have the time to start rotting too much.