Add a dc current source with 0mA dc and 1A ac value. Run transient and note down the time period. Do a ac times sim over the time period. Now you can find ac voltage on that node over the period. You have voltage and current hence find the cap.
Ps: cap corresponding to a node in ring oscillator varies over the period, for the oscillation frequency calculation, I would suggest use mid rail cap.

Best Regards,
Raj.