Who are the clients of my shared iTunes library?
From
Macworld:


  Keeping tabs on who’s listening
  
  The Sharing pane in iTunes’ preferences will tell you how many users are connected to your shared library. But what it won’t tell you is what they’re actually listening to. To find out, open Activity Monitor (/Applications/Utilities), highlight the iTunes entry, and then click on the Inspect button in the toolbar. Now click on the Open Files And Ports tab and then scroll to the bottom of the list in the window below. If you’re listening to something in iTunes, your current song will show up first, but anything someone connected to you is viewing or listening to will show up like this:

 192.168.1.3:daap->192.168.1.8:56089
/Volumes/Backup/iTunes Music/Brian Eno/Another Day On Earth/01 This.m4a
  
  In this example, the person is connected from IP address 192.168.1.8 and is listening to Brian Eno’s “This.”