There are less than a handful of papers in the literature on the phase measurement of surface Second Harmonic Generation (SHG) from liquid interfaces, in sharp contrast to tens of the works on such measurement of films on solid substrate surfaces. Even though the SHG phase measurement is very important for obtaining structural and spectroscopic information, such as molecular orientation, at the liquid interfaces, experimentally there are many difficulties due to the intrinsically weak SH signal of liquid interfaces. Using a femtosecond pulsed laser and photon counting system, we have demonstrate the ability to measure the SH phase for the non-resonant neat air/water interface, which is clearly among the systems with the smallest surface SH susceptibility, and is well below the detection limit of experimental setups in most laboratories. After clarifying some confusions about the reference standards for absolute SH phase measurement using the α-quartz crystal, we used air/water interface as an intrinsic SH phase reference standard to measure the SH phases of some air/pure-liquid interfaces, and adsorbed molecular layers at the air/water interface. These results demonstrate the effectiveness of this very sensitive technique in obtaining molecular structural and spectroscopic information of liquid interfaces.