Listen for the sound of water running or water dripping. As the ice buildup melts inside the refrigerator and freezer, the water runs through a drainage system and ends up in the drain pan, where it evaporates.
Place a kitchen thermometer inside the refrigerator. When the thermometer rises above the thermostat setting, it indicates that the heater coils turned on to start the defrosting process. This method is not 100 percent accurate, as leaving the door open for long period will have an effect on the temperature inside the refrigerator.
Complete a visual inspection of the ice buildup in the freezer. If the ice appears wet or slushy, it indicates the refrigerator is in defrost mode.