Well it's not exactly straight forward!
First enable the console, this thread explains how to do it.
viewtopic.php?f=14&t=1982
Then enable the developer commands,
viewtopic.php?f=14&t=1630&p=16085#p15791
From here there's a host of special key commands (in the previous thread) The one I used for this was F11, this brings up the developers map as shown in this thread,
viewtopic.php?f=14&t=2277
Once you have it up, you get a list of every item on the level, plus a little dot to show where each one is, you still have to highlight them to figure out which dot is which though. Whatever you do stay away from the m command...
Disclaimers, this is somewhat complex, and you can wreck all sorts of things, so try at your own risk!
Daniel.