We present a thorough study of the target-cleaning phase to estimate the healthiness of the target in a direct current (DC) magnetron sputtering deposition. The study is based on real-time plasma monitoring by means of optical emission spectroscopy during a traditional cleaning phase in an Ar atmosphere. In this work we demonstrate that intensities of Ar emission lines are sufficient indicators of the target cleanliness degree. To derive these results SiOxNy thin films were grown by reactive DC magnetron sputtering on silicon wafers for different deposition configurations of Ar, O-2 and N-2 fluxes. Refractive index of the resulting films is measured by in-situ spectroscopic-ellipsometry. A simple but robust estimator is used to determine the time when the target is ready to start deposition. Hence, this approach can be suited for an industrial environment since the time invested in the cleaning phase can be minimized avoiding the waste of material and time.