Ooh yes, that looks like a good start. Presumably that's measuring the mug warming up rather than down (hence, presumably, your comment about needing to tweak it a bit), but it looks like basically the right thing.

Now all it needs is the extra mechanism for detecting an empty mug – preferably without dangling something grubby in the drink. I feel there ought to be something clever along those lines: some sort of micro-seismic detector, perhaps, that wobbles the mug slightly and then 'listens' for ripples bouncing back from the far side to see if there's still liquid in there.