Hi Tron,

Since output impedance is a small signal parameter, first thing you have to do is set the operating bias point for your amplifier. After having done this next step is to apply an AC current with an ideal current source into the amplifier's output and measure the resulting voltage, swept over frequency. The ratio between the voltage and your  AC current source will be your output impedance over freq.
When you are setting the bias point you can leave the AC current source in the output connected, since it will not change the amplifier's operating point

Best
Tosei