We investigate the spinodal decomposition of a polymer mixture, at both critical and off-critical compositions, using atomic force microscopy. Phase separation in the bulk is imaged using tapping mode on the surface of microtomed samples. The generated surface profiles, revealed in height images, are analyzed according to their in-plane spinodal morphology and their (perpendicular) height distribution. The former is characterized in terms of the periodicity of the structure and volume fraction of coexisting phases, both in the percolation and cluster regimes. The average height profiles are shown to be bimodal with a height step, Deltah, ranging from 1 to 7 nm, for the temperature quench depths spanned. Deltah is time-independent but depends linearly on annealing temperature and therefore on the composition difference between coexisting phases. This temperature dependence allows us to extrapolate to the mixture's critical temperature. A blend of tetramethyl bisphenol A polycarbonate and polystyrene was employed for this demonstration.