We developed a procedure for a fast and non-destructive determination of the Ge-content in virtual Si1-xGex-substratesin the range from x = 0.2-1. The virtual substrates were grown in our MBE-system. The reflectivity of SiGe substrates depends on the Ce content x and on the crosshatch surface corrugations. The extremes in relative reflectance values are rather insensitive to surface corrugations contrary to the absolute values which depend significantly on the surface morphology. The reflectance of films was measured ex situ with a commercial measuring system (NanoSpec). This system measures relatively to a measurement standard wafer (Silicon-wafer) over wavelength range from 370 to 800 nm. The reflection graphs have maxima in the range from 410 to 590 nm depending on the Ge-content. Results obtained by these measurements with the NanoSpec measuring system were in a good agreement with SIMS-analysis and XRD data.