nmos kn
due to short channel effect and other parameters, this value is not very accurate so it's utility is limited in professional design. but I know what you mean - it's nice to have a starting value for some paper calculation!
so just build a current mirror with a fairly large area and in simulation, check the dc operating point. From this, you can get Vdsat and GM from sim value to back-calculate k'p and k'n.
For example, in a 0.5um process I had been using recently, I built mirrors of W=5um, L=8um and extracted values of 40uS for k'p and 128 for k'n. Using this for other circuits (op amp) gave a good starting point on sizing the class-AB control for my desired vdsat.
good luck!