Calculations of soft-x-ray emission spectra from Si(100) layers buried in GaAs are reported. The local densities of states for Si in As and Ga sites are found to be very different. By comparison with experimental data, this difference allows us to determine the relative amounts of Si in the two types of sites. In the case of a single Si layer we find that 63 (±5)% of the buried atoms are in Ga sites.