A new method is developed to estimate effective actuator types and locations for controlling compressible, viscous flows using linear feedback. Based on an analysis of the eigensystem structural sensitivity of the linearized compressible Navier-Stokes operator about a steady baseflow, the forward and adjoint global modes are used to estimate where control actuation would be most effective based upon the eigensystem, and what type of actuation (e.g. mass, momentum, energy) would be most effective. This procedure provides an assessment of actuation effectiveness and the quantities to sense to adjust the actuation. The method is demonstrated for the case of a separated boundary layer in a twodimensional Mach 0.65 diffuser. For sufficiently low Reynolds numbers global stabilization of the flow is achieved, which is confirmed with a direct numerical simulation.