IntControl(84, p1, 0, 0, 0)
Itemize desktops, or windows on specified desktop.
If p1 is a blank string (""), this returns a tab-delimited list of desktops, or a blank string ("") if the function fails.
If p1 specifies a desktop name, this returns a tab-delimited list of "Window ID's" of all top-level windows on that desktop, or a blank string ("") if the function fails.