XmGetFocusWidget man page
XmGetFocusWidget — Returns the ID of the widget that has keyboard focus
#include <Xm/Xm.h> Widget XmGetFocusWidget( Widget widget);
XmGetFocusWidget examines the hierarchy that contains the specified widget and returns the ID of the widget that has keyboard focus. The function extracts the widget ID from the associated Shell widget; therefore, the specified widget can be located anywhere in the hierarchy.
Specifies a widget ID within a given hierarchy
Returns the ID of the widget with keyboard focus. If no child of the Shell has focus, the function returns NULL.