XmIsMotifWMRunning man page
XmIsMotifWMRunning — A function that determines whether the window manager is running
#include <Xm/Xm.h> Boolean XmIsMotifWMRunning( Widget shell);
XmIsMotifWMRunning lets a user know whether the Motif Window Manager is running on a screen that contains a specific widget hierarchy. This function first sees whether the _MOTIF_WM_INFO property is present on the root window of the shell's screen. If it is, its window field is used to query for the presence of the specified window as a child of root.
Specifies the shell whose screen will be tested for mwm's presence.
Returns True if MWM is running.