A sensor assembly for determining whether a hermetically sealed cavity between opposing substrate wafers in a wafer level packaged (WLP) chip is leaking. The sensor assembly includes a thermal insulating layer provided within the cavity, and a heater and temperature sensor deposited on the insulation layer. The thermal insulating layer is made of a suitable dielectric that is compatible with WLP and MMIC fabrication processes and can be, for example, benzocyclobutene (BCB) or polyimide. The sensor is responsive to a current that heats the thermal insulation layer so that heat dissipated by the thermal insulation layer is drawn away by gas between the layer and the substrate that determines the temperature of the sensor, which is detected.