For pure substances like steam the specific entropy is included in the steam tables similar to specific volume, specific internal energy and specific enthalpy.
The specific entropy of saturated liquid water (x=0) and dry steam (x=1) can be picked from steam tables. In case of wet steam, the actual entropy can be calculated with the vapor quality, x, and the specific entropies of saturated liquid water and dry steam:
swet = ss x + (1 – x ) sl
swet = entropy of wet steam (J/kg K)
ss = entropy of “dry” steam (J/kg K)
sl = entropy of saturated liquid water (J/kg K)