gah. its tcpdump not tcp dump :s.
Should already be there, if not apt-get it.
sudo tcpdump 'host master.tremulous.net' should work, and give output like:
12:17:09.340505 IP lolcat.30723 > master.tremulous.net.30710: UDP, length 24
12:17:09.470155 IP master.tremulous.net.30711 > lolcat.30723: UDP, length 20
12:17:09.470267 IP lolcat.30723 > master.tremulous.net.30711: UDP, length 142
Which is us sending our IP, the master asking for info, and us sending info. Its that last step that gets us on the list.