One trick.
Place voltage source with AC magnitude = 1V in series with output between output and load. Do AC analysys and measure current flowing though output terminal - Iout.
The output impedance Zout=1V/Iout.
supposing your amplifier has a one dominant pole, you can plot the Bode's diagram of the opamp gain and then calculate:
Rout=Aol/Gm
where Aol is the DC gain you can read from the Bode's plot
and Gm is the global transconductance of the opamp (which value also depends on with how many stages the opamp is built-up).