As micro-optical components are introduced into optical systems, accurate surface characterization becomes important. We describe a method for quantitative evaluation of surface deformations based on Foucault's knife-edge test. By measurement of local slopes, the surface shape of each mirror in a micromirror array has been reconstructed with a subnanometer accuracy. In addition to low-order deformation (tilt, curvature, astigmatism), each mirror is seen to be palm-tree shaped. This may be explained by strain relaxation in the fabrication process. Measurement on a conventional concave mirror confirms our method.