lsof |grep iTunes |grep TCP

You can also find out what they are listening to by doing

lsof | grep mp3

Now how about for a little fun , say your colleague is listening from a windows machine

nmblookup -A 192.168.0.23

gives you their netbios name , next on use that name with :

echo “Ejoying songname John ?” | smbclient -M THENAMEHERE

shows a dialog like this smack middle in their windows screen

Note that you can send messages like this to any computer you see in finder’s SHARED sidebar , just don’t abuse it , it will stop being funny.