This is from a report I wrote for university a while ago. We used ProMOST but the basic principle I followed is you take the derivative of Id with respect to Vgs to obtain gm. IIRC you can do this by doing a DC sweep on Vgs and using d(I(drain_node)) in spice. Then once you know gm, Vgs, Vth, W/L, you can solve for uCox and that will then allow you to do hand calculations around that bias point.