Since you are looking for a substantial and rapid change in temperature, almost any small, fast responding (thermal time constant measured in seconds) thermistor mounted on the radiator will register this.
In lieu of a thermistor you can also use a tiny silicon diode in series with an 8.2K resistor in series with a 9v battery. One degree C increase means a 2.2 mV decrease in the diode voltage drop.
In lieu of a thermistor you can also use a tiny silicon diode in series with an 8.2K resistor in series with a 9v battery. One degree C increase means a 2.2 mV decrease in the diode voltage drop.