This function queries the pointer-conversion procedure associated with a window.

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

HWND    hwnd;     /*  Window handle. */
PFN     thunkpr;  /*  Pointer-conversion procedure identifier. */

thunkpr = WinQueryWindowThunkProc(hwnd);