Hi Tim

first off, you can already see the cap you're setting in the schematics, right?  :D
For MOS varactors, you specify the W/L  and you can see the cap value
well, cap vs. freq will be constant but impedance vs. freq is what you're looking for..uh?
If you go by S-parameters, you can get Zin/Zout as function of frequency 'n then calculate cap.

Or, simply run an ac sim with a standalone varactor 'n you can characterize Cap vs. voltage bias which will be required for your VCO tuning.

Hope this helps

If small signal SP is performed, use the following equation to get C vs freq curve with the calculator in Spectre.

C=imag(y(1,1))/2/pi/xval(imag(y(1,1)))

Maybe more accuracy results can be obtained by PSS+PSP.