One of the Lockergnome properties has this article on displaying hidden devices in Device Manager. Displaying non-plug-and-play devices is fairly easy: right click My Computer -> Properties. Click on the Hardware tab and select Device Manager. In the View menu, select Show Hidden Devices and there they are.
Devices that are defined but not currently present (e.g. MP3 players or USB drives) is harder. You must create an environment variable that has a particular value in order to force them to show up. Again, My Computer -> Properties and go to the Advanced tab. Select Environment Variables and create a new System variable called devmgr_show_nonpresent_devices. Set its value to 1 and click OK to save it and OK to exit the System Properties dialog. Now you can open Device Manager and see everything, You’ll have to select View -> Hidden Devices but, once you’ve done that, both ghosted and hidden devices will show up.