We demonstrate the on-chip detection of light using photosensitive detectors based on quantum dot (QD) micropillar cavities. These microscale detectors are applied exemplarily to probe the emission of a monolithically integrated, electrically pumped whispering gallery mode microlaser. Light is detected via the photocurrent induced in the electrically contacted micropillar detectors under reverse-bias. In order to demonstrate the high potential and applicability of the microdetector presented, we determine the threshold current of an integrated microlaser to be (54 ± 4) μA, in very good agreement with the value of (53 ± 4) μA inferred from optical data. Within this work we realize the monolithic integration of a laser and a detector in a single device operating in the regime of cavity-quantum electrodynamics. Our results thus advance the research on microscale sensor technology towards the few-photon quantum limit and pave the way for on-chip opto-electronic feedback experiments.