Abstract: A method and apparatus for inspecting the location and orientation of a sensor assembly in a tray-sensor arrangement adapted to be placed in a microwave oven. The sensor assembly which includes a ferromagnetic element that absorbs energy when placed in the oven and changes its characteristics when a predetermined amount of energy has been absorbed, is tested in the completely assembled tray by accurately positioning the tray in predetermined relation to an oscillating circuit whose operating frequency is responsive to the presence of the ferromagnetic sensing element. The output frequency of the oscillating circuit indicates whether or not the sensor assembly has been properly assembled and positioned on the tray. The oscillator circuit includes a frequency determining element which produces a distinctly different frequency when the sensor is not properly positioned in the assembly or is omitted from the assembly.