DC offset arises due to imperfections such as device mismatch and LO imbalance. Therefore before you can simulate it you need to introduce some kind of mismatch such as LO phase and amplitude mismatch or transistor width/length mismatch. You may want to check out

"DC Offsets in Direct-Conversion Receivers: Characterization and Implications" by R. Svitek and S. Raman, IEEE Microwave Magazine.

Anyway I think you can just use a PSS analysis. Also remember for balanced topologies, some of the mismatches can cancel each other off...

Aaron