there are two main contributors to the offset,
the systematic offset and the random offset,
by considering a fully differential amplifier, everything is symetrical, you may get a small enough systematic offset,
whereas random offset depends on transistors' matching, so it's never 0 nor very small to pV range, it's still in the mV range for current CMOS technologies.
Monte Cralo is one solution (the best).
The other way is to put DC sources at each pair of transistor. Get matching data from the fab and see the influence.