X-ray fluorescence (XRF) monitoring of characteristic peaks while etching thin-film layers can reveal coverage defects and thickness nonuniformity in the top film. To measure coverage and uniformity while screening candidate layer materials and processes, the candidate layers may be formed above an underlayer of a different composition. A wet etchant that selectively etches the underlayer faster than the candidate layer is applied to the candidate layer, and the XRF spectrum is monitored. Pinholes, cracks, islands, and nonuniform thickness in the candidate layer produce characteristic features in the time-dependent behavior of XRF peaks from the underlayer and/or the candidate layer. “Etch/XRF” tests can be used to rapidly and objectively identify the most uniform contiguous candidate layers to advance to further screening or production. XRF may also be calibrated against a known thickness indicator to detect the approach of a desired endpoint in an etch process.