You can determine what screen resolution you are in by using a command called xdpyinfo
I may be wrong, but I don't think XFree86 supports mode switching (at least in a stable release). I read that it is comming, but not sure how it will work when released.
It isn't quite mode switching, but in your XF86Config-4 (assuming you are using XFree86 4.X), under the "Display" tag you can specify "Modes" like this:
Modes "1024x768" "800x600" "640x480"
Then you can use Ctrl + Alt + + (Press and hold Ctrl, Alt, and +) Wanted to explain that 'cause it looked a little wierd.
Ctrl + Alt + - (Press and hold Ctrl, Alt, and -) to switch resolutions of the same screen.
This doesn't resize the desktop. It only changes resolutions.