The easiest is to do an AC small-signal sweep with bias voltage as secondary sweep.
M1 vss gate vss vss NCH L=xxx W=www
Vac gate vbg AC 1m
Vbias vbg vss DC bias
.AC dec 20 fstart fstop
+SWEEP bias 0 3 50m
... and observe the current in Vac.
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