If it's just a C model, then:
- AC analysis
- Connect a current source, 1A, 0 deg, to the capacitor. Ground one side.
- The not-ground node has voltage V = Z*I. Read off real (resistance) + j*imag (reactance) components directly.
- Q = reactance / resistance.
- If desired, take 1 / (2*pi*imag(VC)*frequency) to read off capacitance as well.
This also works if the C is dependent (nonlinear), in which case you get the small signal approximation (which is what AC analysis is). You may want to do a parameter sweep, to measure the capacitance at other bias conditions.
This does not work if the C is equivalent over some time-varying function, like a switched-capacitor filter, for which you need to do the transient analysis and set up all your instruments (source, quadrature mixer, detector..) just as you would measure the resistance and reactance IRL.
Tim