  We report on a new method to characterize nano-hydrodynamic properties at the
liquid/solid interface relying solely on the measurement of the thermal motion
of confined colloids. Using Fluorescence Correlation Spectroscopy (FCS) to
probe the diffusion of the colloidal tracers, this optical technique
--equivalent in spirit to the microrheology technique used for bulk
properties-- is able to achieve nanometric resolution on the slip length
measurement. It confirms the no-slip boundary condition on wetting surfaces and
shows a partial slip b=18 +/- 5 nm on non-wetting ones. Moreover, in the
absence of external forcing, we do not find any evidence for large nano-bubble
promoted slippage on moderately rough non-wetting surfaces.
