How to simulate the even harmonic impedance of the RF balun in ADS?

By using s-parameters..

but how to distinguish the fundamental and the harmonic impedance?

If you do a s-parameters simulation, you can plot s-parameters on Smith Chart and you will read the impedances at the frequencies where you are interested in.
For instance, let say your Balun works at 1GHz and you would see the input/output impedances at 2GHz.You do a s-parameters simulation from 1GHz to 2GHz and you put the marker on the Smith Chart at 2GHz and read the impedances.