This function determines if a window handle is valid.

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

HAB     hab;   /*  Anchor-block handle. */
HWND    hwnd;  /*  Window handle. */
BOOL    rc;    /*  Validity indicator. */

rc = WinIsWindow(hab, hwnd);