An interferometric method to simultaneously test plane parallel plates both for planarity and parallelism of end faces is reported. Measurements are taken by insertion of the plates in a reference cavity. The optical configuration uses a standard programmable interferometer, with an external right-angle prism folding back the probe beam. The prism error is determined by cavity calibration, and is subtracted at data reduction. The measuring procedure is discussed in detail, and results of a laboratory demonstration are presented.