Many natural and industrial processes involve fluid flow at a three phase interface between liquid, solid and gas. When the liquid is forced to move rapidly over the solid surface, the physical mechanisms governing contact line motions are multi-phase and multi-scale and as a consequence the dynamics can be complex. We perform direct observations of the wetting phenomena at the interface of liquid droplets impacting on a rigid flat surface using high resolution microscopy and high speed imaging. Upon impact of a liquid droplet on a smooth solid, a thin air film is formed and squeezed in between the two. Depending on the impact parameters, the air film can remain stable and facilitate the rebound of the droplet, or liquid-solid contact occurs that binds the droplet to the solid surface, leads to subsequent spreading or splashing. In order for the liquid-solid contact to occur, the air film must rupture under the impacting droplet. The physical mechanisms responsible for the rupture of the air film have not yet been conclusively determined; in particular a quantitative test for the theory of air film rupture is lacking. Currently, the mechanism assumed to be responsible for liquid-solid contact formation is an interfacial forces-driven destabilization of the initially flat liquid-air interface. The liquid-air interface is destabilized when the attractive stresses (e.g., van der Waals forces or electrostatic forces) overcome the stress associated with deforming the surface due to surface tension. This instability is characterized by a critical air film thickness (h*) at which the film destabilizes and wetting initiates. We carefully measured the air film thickness at which the droplet rebounds for water-glycerol solution and silicon oil droplets with similar viscosities on atomically smooth mica surfaces. h* for the silicon oil (~ 30 nm) is smaller than that of the water-glycerol solution (~ 60 nm); however, the local minimum air film thickness immediately prior to contact formation, is approximately the same value for both liquids (~ 20 nm).