If you are using Cadence software, you should add port in input and output and in ADE window you select analysis in "s parameter" and give appropriate value. Then select netlist and run the simulation. After simulation you go to result then direct plot. you should select s parameter. Now you can find S11, S12, S21 and S22.

The most simple is using 4-ports VNA in differential mode.

Dear Abhishek,

Thanks for your answer, But my doubt is how to connect port to differential signals?. Usually if port analysis is used for single-ended signals. As per my knowledge we have to use a converter(to convert differential to single))  to connect port to differential signals. Do you have any idea on this?