The following shows what DE/WMs are available:
Code:
james@somecomp:~$ ls -l /etc/X11/xinit/
total 28
-rw-r--r-- 1 root root 321 Mar 16 2003 README.Xmodmap
lrwxrwxrwx 1 root root 11 Oct 2 2010 xinitrc -> xinitrc.kde*
-rwxr-xr-x 1 root root 651 Feb 27 21:59 xinitrc.blackbox*
-rwxr-xr-x 1 root root 659 Feb 27 22:03 xinitrc.fluxbox*
-rwxr-xr-x 1 root root 644 Feb 27 22:07 xinitrc.fvwm2*
-rwxr-xr-x 1 root root 627 Jan 7 21:21 xinitrc.kde*
-rwxr-xr-x 1 root root 725 Dec 1 2009 xinitrc.twm*
-rwxr-xr-x 1 root root 894 Feb 27 22:06 xinitrc.wmaker*
lrwxrwxrwx 1 root root 28 Mar 31 00:27 xinitrc.xfce -> ../../xfce/xdg/xfce4/xinitrc*
That xinit links to xinitrc.kde shows that when xwmconfig was last run as root the system default DE/WM was set to KDE. If a user runs xwmconfig and selects a different DE/WM then 'startx' for that user will start the selected DE/WM.
You can also start any one of them with a command of this form:
Code:
startx /etc/X11/xinitrc/xinitrc.xfce
That will start XFCE4.
I can't take any sort of credit for any of that, it all came from these forums, somewhere.
Naturally, this is limited to non-GUI runlevels (like 3). As you're all well aware (for completeness), selecting a different DE/WM from KDE is simply clicking on the button on the login screen. I'm not sure how to do that for xdm, maybe you can't.
[edit: last paragraph added]
[edit2: hrmph. I seem to have answered a slightly different question that what was asked. ;v) ]