XtBuildEventMask - retrieve a widget's event mask
#include <X11/Intrinsic.h>
EventMask XtBuildEventMask(Widget w);
The XtBuildEventMask function returns the event mask representing the
  logical OR of all event masks for event handlers registered on the widget with
  XtAddEventHandler and all event translations, including accelerators,
  installed on the widget. This is the same event mask stored into the
  XSetWindowAttributes structure by XtRealizeWidget and sent to
  the server when event handlers and translations are installed or removed on
  the realized widget.
XtAddEventHandler(3)
X Toolkit Intrinsics - C Language Interface
Xlib - C Language X Interface