Well, if you put a lid on a saucepan (and you should, it conserves energy and makes water boil faster) you can detect it clattering when the water starts to boi

Well, if you put a lid on a saucepan (and you should, it conserves energy and makes water boil faster) you can detect it clattering when the water starts to boil! So you don't need any optical input, a microphone will suffice ― which is cheaper, too. Filter out the low frequencies of water humming, amplify the rest, and you got a (not-so-nice-sounding, because it rattles, not whistles) boiling point alarm!