Magnetic properties of Ge1-xMnx epitaxial layers with a Mn content of a few percents are substantially influenced by inhomogeneities in the distribution of Mn atoms in the Ge lattice. Depending on the substrate temperature during molecular-beam epitaxial fabrication, apparently cubic, coherent Mn-rich clusters or incoherent precipitates consisting of the hexagonal, intermetallic Mn5Ge3 phase can occur in a defect free, diamond lattice Ge matrix. In this work, we apply synchrotron x-ray diffraction in grazing-incidence geometry to probe the diffuse scattered intensity of the distorted Ge host lattice. Based on a theoretical description of the scattered intensity we derive quantitative information on the lattice mismatch between the Mn inclusions and the Ge lattice, as well as on the average size of the inclusions and the average Mn content within the inclusions.