In this study, we report on a methodology based on reverse and forward current-voltage curves (I-V) and on Degree of Polarization (DoP) of electroluminescence measurements on 980 nm laser diodes chip-on-submount (CoS) for the improvement of screening tests. Current-voltage curves are performed at reverse bias up to breakdown voltage (VBR) using both a high current accuracy (< 1 pA) and high voltage resolution (< 10 mV) at different submount-temperatures (20-50°C). The DoP of luminescence of such devices, related to strains in materials and effect of shear strain on the birefringence, is calculated from the simultaneous measurement of TE (LTE) and TM (LTM) polarized light emissions. We observe that application of high reverse voltages occasionally produces significant micro-plasma (MP) pre-breakdown on reverse I-V characteristics as recently observed in InGaN/GaN LEDs and assumed to be a response of electrically active defects. Comparisons between breakdown voltages and number of MP, and changes of leakage current at low forward voltage (< 0.1 V) are considered. DoP measurements are also analyzed versus temperature. Finally the usefulness of these measurements for effective screening of devices is discussed.