The built-in strain in self-assembled quantum dots has large impact on their physical properties, but both its average value and degree of anisotropy are often unknown. The authors demonstrate that the pressure coefficient of optical phonons might be used as probe for the strain status of the dots. This method was applied to the case of Ge dots capped with Si layers of different thicknesses. The authors observe a transition from a strictly biaxial stress situation for uncapped dots to a status of quasihydrostatic strain for cap-layer thicknesses larger than a critical value of the order of the dot height. © 2007 American Institute of Physics.