We investigate the properties of nanometric liquid films with a new non invasive technique. We measure the spontaneous thermal fluctuations of the free surfaces of liquids to probe their hydrodynamic boundary condition at a solid wall. The surface fluctuations of a silicon oil film could be described with a no-slip boundary condition for film thicknesses down to 20 nm. Oppositely, a 4 nm negative slip length had to be introduced to describe the behavior of n-hexadecane, consistently with previous Surface Force Apparatus data on the same system. Our results demonstrate that at vanishing flow a nanometric solid-like layer close to the wall may exist according to the nature of the liquid.