Reflection second harmonic generation experiments were performed on z-cut congruent lithium niobate crystals (LiNbO3) to reveal the interfacial layer symmetry as the crystal is rotated around the z axis. To suppress the bulk contribution, the fundamental wavelength was selected to be 532 nm, resulting in second harmonic generation at a wavelength within the absorption region of the crystal. The polarity of the direction of the y-axis was determined from second harmonic generation data and used to show that this direction also inverts during domain inversion.