A probe includes a sensor and a support. The sensor is operable to provide an indication of a thickness of a frozen substance that has accumulated between the sensor and a cooling fin of a cooling unit, and the support is operable to hold the sensor spaced apart from the cooling fin. When a defrost controller uses such a probe to monitor an amount of frost build up on the fin or fins of a cooling unit (e.g., a refrigeration unit or freezer unit), the defrost controller can initiate a defrost cycle only when warranted, e.g., only when the thickness of the built-up frost equals or exceeds a threshold thickness.