temp sensors aren't TOO expensive.
http://www.aeroforcetech.com/Temp_sensor.html However, I still think that by the time this sensor indicates a problem, you're already in trouble.
It seems like a flow sensor would be cheap and easy (may not be in reality, but seems like it should be). We don't care about the calibration (to determine the ACTUAL amount of liquid flowing), we just need something like a pinwheel. If the axle is rotating, we're fine. If it slows below a certain threshold, something is blocking flow, so turn on a warning light.
Anyone know of a connector that has something in it that could be used like a pinwheel?