@User I typically just use a simple spice simulation to derive equivalent capacitances, e.g. sweep gate voltage from 0->Vdd in a transient simulation and look at currents in all four nodes of the transistor. The currents are proportional to the capacitance as dV/dt is constant. This will also show reduced capacitance for gate voltage around the threshold voltage. If you want averaged capacitance you can integrate the current over the full sweep and divide by Vdd.