We have studied the potential of friction and stiffness measurements with high spatial resolution for the surface characterization of glassy polymers. We present experimental evidence for quasi-chemical sensitivity on a heterogeneous surface consisting of polystyrene islands on a poly(methyl methacrylate) base layer. Although similar in their bulk mechanical properties, the two polymers are easily distinguished by their different nanomechanical behavior. As an example, we characterize the domain pattern formed after spinodal decomposition of a symmetric blend of the two polymers.