In this paper, we propose an in-service method to simultaneously monitor both nominal and effective values of differential group delay (DGD) in wavelength-division multiplexing (WDM) optical communication systems, in a per channel basis. The method is based on coherent heterodyne detection of the optical signal. We have demonstrated that the technique is capable to recover nominal DGD values from 0 ps to 90 ps while, at same time, to provide the effective DGD parameter, related to the impairment of optical channels. The relationship between the Q factor and effective DGD was also demonstrated, both numerically and experimentally, for distinct nominal values of DGD inserted on the system, by varying the state of polarization (SOP) of the optical signal at the input of the DGD element.