Liquid crystal spatial light modulators (SLM) often suffer from defects that need to be compensated for demanding phase modulation applications. Usually the calibration map used to correct the SLM are determined at the factory or on a dedicated optical bench, but either way the measurement is done away from the experimental setup. So this correction map can’t reflect any temporal modification of defects, or correct for defects induced by the experimental setup itself or the environment. With a liquid crystal SLM, the read-out beam phase is modulated by tuning locally the birefringence. We present here a method where we record the birefringence map with a second superimposed light beam which uses the SLM in intensity modulation. In a first experiment we use the birefringence map to deduce the complete phase response of the SLM and optimize its parameters. In a second experiment we demonstrate the correction of externally induced defects: after a comparison between the measured and desired birefringence maps, SLM defects are compensated via a feed-back on the addressed hologram. As SLM monitoring is done in-place we can control time-dependant defects like those induced by a powerful read-out beam or a thermal drift. This method allows us to measure the defects of the SLM with spatial and phase resolutions comparable to interferometric methods. As it relies on polarization modulation, vibrations and misalignments are not critical, therefore supplying robustness. Furthermore, this method provides in-situ measurement, so that it’s easy to compensate day to day defects variation or aging. Finally the demonstrated method is a way open to closed-loop phase correction.