This function indicates if a window is a descendant of another window.

#define INCL_WINWINDOWMGR /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HWND    hwnd;        /*  Child-window handle. */
HWND    hwndParent;  /*  Parent-window handle. */
BOOL    fRelated;    /*  Related indicator. */

fRelated = WinIsChild(hwnd, hwndParent);