Balance output impedance measurement

One option is to derive it from a two-port single ended S-parameter measurement, another to use a balun. If the problem is related to large signal behavior of a non-linear output stage (e.g. a transmitter PA), you would want to refer to load pulling instead.

May I know how to derive from two port single ended S-parameter measurement? Thanks.

I think, it can be calculated referring to fundamental S-parameter properties. But I don't have my RF books at hand now.