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Put in series with the voltage source that provides the reverse bias:
An AC source with low output voltage (for example 50 mV or less)
a Current meter (for determining the AC current through the varactor).
Run a transient analysis and observe both current and voltage (if current waveforms doesn't look sinusoidal, reduce amplitude of AC source). From the amplitudes and phase you can determine |Z| and Arg(Z). From here you can calculate the values for an RC series circuit based on your frequency used.
You may also run an AC analysis.
You may omitt the current meter by selecting the variable that shows the current through the voltage source or diode.