XPGetWidgetGeometry

WIDGET_API void       XPGetWidgetGeometry(
                         XPWidgetID           inWidget,    
                         int *                outLeft,    /* Can be NULL */
                         int *                outTop,    /* Can be NULL */
                         int *                outRight,    /* Can be NULL */
                         int *                outBottom);    /* Can be NULL */

This routine returns the bounding box of a widget in global coordinates. Pass NULL for any parameter you are not interested in.