This function determines whether any part of the window hwnd is physically visible.

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

HWND    hwnd;  /*  Window handle. */
BOOL    rc;    /*  Showing state indicator. */

rc = WinIsWindowShowing(hwnd);