Strain engineering is key element for improving MOSFET device performance in today's advanced high performance microelectronics. Strained SiN layers are well established in this regard to generate near surface strain effects for carrier mobility enhancement. Non destructive detection of effects from strain layers has not been resolved yet, especially during the processing of patterned wafers. Since optical reflective measurements are well established for non destructive thickness measurements in semiconductor manufacturing, the signals out of those spectra can be used to investigate stress induced carrier mobility enhancement effects, as shown in this work. This provides direct access to the effectiveness of the applied near surface strain to the carrier mobility. By this novel method, SiN films at different stress levels and under different process conditions are investigated on silicon bulk wafers. On these wafers, the optically detected spectra shift, known as plasmon shift, can be correlated with standard wafer bow stress measurement results. These results are correlated to the electrical performance of microelectronic test structures. Based on this, the relative variation of the stress levels of SiN films after different process treatments, and the impact on carrier mobility in microelectronic devices is shown.