At first you should know the water cut definition

%water cut = vol. of water at std. cond. / vol. of liquid phase at std. cond. *100

therefor you have to flash your stream, maybe by:

1- change the pressure and temperature of the stream to 14.7 psi and 15 °c, or
2- use a separator, delete (not set to 0) the inlet pressure drop, define duty stream and set the pressure and temperature to 14.7 psi and 15 °c for the output streams.

Then go to the liquid phase composition and change the basis to "Liquid Volume Flow", read the H2O and Total values, divide them and multiply the result by 100. You've got the Water Cut now!