Type "switchdesk" in the console. This will give you a selection of Gnome, KDE, TWM, and WindowMaker. (Providing you have installed these) Any other window managers will not show up (AFAIK)
Nautilus in Gnome is not a Window Manager, it is the File Manager, however draws the desktop icons as well. If you start nautilus when in KDE, you will see your Gnome desktop.
I hope this helps