The transistor/FET model parameter extration in ADS

Dear friend, anyone have some ideas howto extract the output/input shunt capacitor in the transistor/FET ac model. can I just need to use a Hamonic Balance to get the frequency response then try to find the capacitors values by fitting curve. My model is need only a input shunt capacitance and an output shunt capacitance with a current/voltage controlled current source. Any idea? thank you