I use the command below when I want to see everything that's on a specific port for either TCP or UDP. The
-n option disables attempting to resolve the IP addresses into domain names, and the
-P disables attempting to figure out the name of a particular port. Also, running as
root will show you more processes than running as a normal user.
sudo lsof -iTCP:53 -iUDP:53 -n -P
This should be possible in a terminal window using the Netstat command.
And if you like the GUI way more:
With Mac OS X 10.5, the /Applications/Utilities folder contains a network utility called: Network Utility, see tab Netstat for these stats presented in a gui application, along with Ping, Lookup, Traceroute, Whois, Finger and Port Scan.