  We suggest and demonstrate a tomographic method to fully characterize
homodyne detectors at the quantum level. The operator measure associated with
the detector is expanded in the quadrature basis and probed with a set of
coherent states. The coefficients of the expansion are retrieved using a least
squares algorithm. We then validate the reconstructed operator measure on
nonclassical states. Finally, we exploit results to estimate the overall
quantum efficiency of the detector.
