This function returns the handle of the window that has the pointer captured.

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

HWND    hwndDesktop;  /*  Desktop-window handle. */
HWND    hwnd;         /*  Handle of the window with the pointer captured. */

hwnd = WinQueryCapture(hwndDesktop);