This paper presents a general, novel, and accurate method to determine the shape of the temperature-entropy (T-S) vapor saturation curve for fluids, based on Lee-Kesler's version of the corresponding states principle. The slope of the T-S vapor saturation curve and the location of isentropic points are successfully predicted for any fluid as a function of only three input variables: acentric factor (omega), ideal-gas ratio of specific heats at the critical temperature (k(c)), and the exponent of the heat capacity ratio vs temperature power relationship (m). The proposed method is then applied to a set of 120 commercially available fluids, and the results are validated with experimental data. As the method effectively distinguishes among dry, wet, and isentropic fluids, a simple semiempirical equation establishing the frontier between wet and dry fluid regions is discovered. The developed method can be used as an effective tool for working fluid selection for organic Rankine cycles (ORCs), refrigeration cycles, and heat pumps. It also has the potential to aid the development of new fluids with a proper set of characteristics for specific applications.