Hi.
The following formula is from the 2nd chapter of Razavi's book, Analog CMOS IC Design. But note that this formula isn't correct for submicron devices :
ft=gm/2pi(Cgd+Cgs) = µn/2pi * (Vgs-Vth)/L^2
and in HSpice, I think you can use a test circuit. you should use a small-signal current source for example with 1V ac and use an AC analysis to find the frequency where the current gain of the device drops to unity.