A microwave oven includes a cooking cavity having an opening, a source of microwave radiation that transmits microwaves into the cooking cavity, a door positioned adjacent the opening and movable between an open position where the cooking cavity can be accessed through the opening and a closed position where the cooking cavity is inaccessible through the opening. The door further includes a transparent glass panel where the cooking cavity is viewable through the door when the door is in the closed position. A conductive metal transparent coating on at least one surface of the transparent glass panel attenuates microwave transmission from the cooking cavity through the door. The conductive metal transparent coating has a sheet resistance and is electrically grounded. A circuit is connected to the transparent coating that measures the sheet resistance of the transparent coating.