You could export tabular data to Excel and fit the sqrt(Id)
vs Vgs line. Or you could just call some arbitrary current
level your "VT" (but this would not be the VT you want to
insert in a model card, exactly). There's a bunch of VT
extraction methods, giving slightly different results and
resiliencies to measurement conditions & device nonidealities.
What your use is for the VT value, might indicate which one
you prefer (process control in-line wants different things
than SPICE modeling, etc.).