A method for testing a surface of a sample includes irradiating the surface at a grazing incidence with a beam of radiation having a focal region, whereby the radiation is reflected from the surface. At least one of the focal region and the sample is adjusted through a plurality of adjustment stages within an adjustment range so as to vary a location of the focal region relative to the surface. Respective angular profiles of the radiation reflected from the surface are measured at the plurality of adjustment stages, and the angular profiles are compared in order to select an adjustment within the range at which the surface is in a desired alignment with the beam.