I am probably too late, but here goes. I hope it helps someone else.
Make a transient analysis with a very slow input signal, switching up and down more than your expected offset. Meaning, if you expect an offset of 5mV, make the differential input go from -10mV to 10mV, and back down.
Then you have to measure your differential input when the output switches from low to high and form high to low, like if you use spice measures:
.measure TRAN triprise FIND v(vid) WHEN v(o)='vdd/2' RISE=LAST
.measure TRAN tripfall FIND v(vid) WHEN v(o)='vdd/2' FALL=LAST
Then you have 'triprise' and 'tripfall' as your two trip points. Their average is your offset, and their difference is your hysteresis width.
This only works for latched, asynchronous comparators obviously.