Usually i do this by simulating the ac loop gain. For this to work you must have a feedback loop that is closed in DC (so the operating point will be calculated correcty) and open in AC. Following example is a loop gain analysis by opening the loop. This is a differential amplifier used in follower configuration (DIFFOUT normally connected to MINUS). The loop is opened with a low pass filter (the RC network) which allows only DC to be fed back to MINUS. Note also the feedback loading effect is added (MINUS gate loading)