Hello Ed,
its actually fairly easy. I use the Comms Panel in VRC to monitor. (you can open it by clicking on the box with the Zulu time in the top left corner)
You can find well detailed information on how to setup the Comms Panel
here.
To determine a controllers voice server and channel, open a text message with that controller by double clicking on the callsign of the controller in the controllers and chat window (open by clicking on "Tools"->"Controller List" in the VRC menu bar) and then double clicking on the empty space where sent and received text messages appear.
Please note to not "prime" or transmit on an active frequency as observer.
Hope this helps.