Use a microphone.  To keep the mic cool, attach it to the vessel
which contains the boiling fluid by using a length of metal - to
transfer vibration but not heat.  Metals with low thermal conductivity,
such as stainless steel will work better than copper of aluminum for this.

As far as monitoring the mic, you might want to use a frequency
to voltage converter and plug that into an analog input on a PIC.
I'm not an analog guru, so that's the best answer I can provide.



> I have read some great suggestions. I have a couple of questions.
>
....
>
> One of Nicholas Irias suggestions I really liked was measuring a high
> frequency noise to determine boiling. What method would you suggest to
measure
> frequency? What sensor could I use? Could you elaborate?
>