The variation in the glass transition temperature of thin polymeric films represents a phenomenon yet to be fully explained. To date, it is widely agreed that it is linked to the interfaces that the film forms with the air and the supporting substrate. Herein, we address one of the main issues regarding the viscoelastic behavior of the region near the free interface of thick polystyrene films where a reduction in the glass transition temperature is expected to occur. We have measured, as a function of the temperature, the elastic and viscous responses of polystyrene films with molecular weights above and below the critical value for the occurrence of molecular entanglement. The experiments have been carried out by means of scanning probe microscopy in a configuration combining the acquisition of force versus distance and indentation versus time curves. We show that the viscoelastic behavior of polystyrene films with thickness down to 30 nm can be successfully evaluated in the time scale from tenths to tens of seconds. In particular, we observe that the viscoelastic behavior of thick film surfaces has a similar dependence from the temperature as the viscoelastic behavior of the bulk, independently if the molecular weight is above or below the critical value. We estimate that the region at the free interface with a reduced glass transition temperature, if present, has a thickness below 3 nm.