A new interferometric technique is described to measure the complex modulation curve of a spatial light modulator. Based on a lateral shear imaging interferometer, it enables the amplitude and phase modulation for several modulation levels to be displayed simultaneously in a single interferogram. As an example of the power of this technique a heuristic optimization of input and output elliptical polarization states for a mostly-phase operation mode was obtained within a few minutes for a commercial twisted-nematic liquid crystal display.