A model for the spreading of a non-polar liquid on a surface within a polar medium is described theoretically, according to Fowkes, Good and Girifalco approximations on interfacial tension. We demonstrate both theoretically and experimentally that surface polarity measurements using the contact angle of two immiscible liquids minimize drastically the measurement error. The present method has been successfully applied to various substrates of variable polarity and overall surface energy. We also demonstrate that this method allows a direct measurement of surface sensitivity to pH.