HWND GetWindow(
HWND hWnd, // handle of original window
UINT uCmd // relationship flag
);

...

If the function succeeds, the return value is a window handle. If no window exists with the specified relationship to the specified window, the return value is NULL.

; callback procedure used in EnableAllOwnedWindows
proc _EnableOwnedProc, hwnd, owner
        begin

        invoke  GetWindow, [.hwnd], GW_OWNER
        or      eax,eax                       ; this 2 lines
        jz      .exit                         ;
        cmp     eax, [.owner]
        jne     @f
        invoke  EnableWindow, [.hwnd], TRUE
@@:
        mov     eax, 1
.exit:                                        ; this line
        return
endp


; callback procedure used in EnableAllOwnedWindows
proc _DisableOwnedProc, hwnd, owner
        begin

        invoke  GetWindow, [.hwnd], GW_OWNER
        or      eax,eax                       ; this 2 lines
        jz      .exit                         ;
        cmp     eax, [.owner]
        jne     @f
        invoke  EnableWindow, [.hwnd], FALSE
@@:
        mov     eax, 1
.exit:                                        ; this line
        return
endp