Buried charges appear for example at the interface of a plasma and a solid. To measure them optical spectroscopy can be used by exploiting their influence on the dielectric function of the material they are buried in. We investigate ATR spectroscopy in a layered system and find the position of certain reflection minima to shift with the charge density. The reflectivity is calculated classically using a continued fraction method that provides an effective dielectric function for the layered system. We consider visible and infrared wavelengths, where different features in the reflectivity turn out to be charge-sensitive. </pre><pre>For visible wavelengths it is the reflectivity dip due to the (surface) plasmon which provides the charge diagnostics whereas in the infrared it is the dip arising from the interaction of the plasmon with the (bulk) phonon resonances of the structure. In particular the latter is rather sensitive to the charge density and thus well suited for robust charge diagnostics.