The invention comprises systems and methods determining residual stress such as that found in interferometric modulators. In one example, a test unit can be configured to indicate residual stress in a film by interferometrically modulating light indicative of an average residual stress in two orthogonal directions of the substrate. The test unit can include a reflective membrane attached to the substrate where membrane is configured as a parallelogram with at least a portion of each side attached to the substrate, and an interferometric cavity formed between a portion of the membrane and a portion of the substrate, and where the membrane is configured to deform based on the residual stress of in the film and modulate light indicative of the amount of membrane deformation.