In radial gradient index rods (GRIN rods) stress birefringence may appear due to the variation in chemical composition. This results in image degradation in addition to that produced by aberrations. We propose an interferometric method to evaluate the effects of birefringence and the aberrations at the same time. It uses the four-step phase stepping technique twice with two different polarizations of the reference beam. The resulting information consists of two phase maps, one containing information about the aberrations and the other about the birefringence effects given as a relative phase delay between the radial and azimuthal components of the test beam. The main feature is that this relative phase delay is evaluated locally over the aperture at the exit pupil.