I only activate my vncserver when i need it.
That is i ssh into my box, run vncserver and that command tells me the desktop number of the new session.
Since i only vnc from windows, i use the port forwarding in putty (ssh client) and hence have a secure VNC session.
[martin@*** ~]$ vncserver
New 'X' desktop is ***.net:2
Starting applications specified in /home/martin/.vnc/xstartup
Log file is /home/martin/.vnc/****.net:2.log
Above the :2 indicates display 2, hence port 5902
I'm not sure how to determine it for already running vnc sessions. sorry.