i didn't have anything in mind in particular, I've seen youtube videos of people using anything.. aluminum foil seems to work maybe some aluminum tape would be better. and stick it to the cup with a couple of wires going to the core
i think the hard part will be other influences on it like touching it with your hand or when drinking and the level changes as the cup tilts etc.
here is a bit of discussion about using capacitive sensors.. the last post someone got it working but I'm not sure how well