A light source is configured to produce light, a waveguide is optically coupled to the light source and configured to direct the light to an intended focus location, and a slider is configured to use the light as an energy source for heating a region of a magnetic recording medium. A thermal sensor is situated on the slider at a location outside of a light path that includes the intended focus location. The thermal sensor is configured for sensing a short time constant change in temperature resulting from light source heating of the thermal sensor, wherein the sensed change in thermal sensor temperature is representative of optical intensity of the light delivered to the intended focus location.