In this paper we propose a soft-X-ray method to characterize dust accumulation or layer formation on a given substrate. The method determines the differential absorption based on the X-ray lines emitted from the substrate by fluorescence as a result of film or powders deposited on the substrate surface. We have chosen to use molybdenum as the material for the substrate because it is used in present-day tokamaks and it is being considered as material for the first mirror. It also offers the advantage of having two strong lines, well separated in energy: the L-shell emissions centered at [approximately]2.3 keV and the K lines at [approximately]17.4 keV. The transparency of the layer can be then measured at 2.3 keV, provided the K line is unaffected. The feasibility of the proposed method was clearly demonstrated in laboratory experiments, providing estimations of the thicknesses that can be detected, for a number of relevant elements for fusion devices (Be, C, Fe, and W).