a method and system for detecting the presence of a frozen deposition at an interface between a substrate of known thermophysical properties and surroundings of unknown thermophysical properties . the system involves applying a predetermined quantity of heat to a temperature sensor positioned at the interface to cause a thermal perturbation , measuring an output signal of the sensor subsequent to applying the predetermined quantity of heat , and comparing one or more measured values of the output signal to one or more reference output level values to determine the presence or absence of the frozen deposition .