We present TWINS (TWo INclining Slits), a method for characterizing phase spatial light modulators (SLMs), inspired by Young interferometry. TWINS is an elegant and versatile approach, using minimal equipment and alignment. It measures phase response locally rather than globally, both horizontally and vertically, with high resolution and at wide angles. It can also measure beam intensity profiles as directly seen by the SLM. TWINS characterizes the anisotropic aberrations in the mainstream models of liquid crystal phase SLMs, which is crucial to improve hologram quality. Compensating for anisotropic aberrations measured by TWINS improved the image quality of planar holograms by 10 dB.