Scanning probe microscopy is used to image nucleation sites leading to dewetting of polystyrene films (<25 nm) from nonwetting Si substrates. Images outline the progression of nucleation until a hole or dry patch is formed. Both spinodal decomposition and particle nucleation are observed to initiate dewetting. Spinodal decomposition appears as surface indentations that grow exponentially with time. Indentation growth rate is measured using an "anneal-quench-image" cycling procedure. Under clean room conditions spinodal decomposition dominates the nucleation process while particle nucleation increases in higher aerosol concentration environments. Upon thermal cycling of the films a third type of nucleation emerges resulting from delamination-induced buckling of the polystyrene film. The three mechanisms of defect formation are described and compared to theories of defect formation during drying of thin liquid films on nonwetting substrates.