Returns the identifier of the specified control
#include <WinAPIDlg.au3>
_WinAPI_GetDlgCtrlID ( $hWnd )
| $hWnd | Handle to the control | 
| Success: | Identifier of the control | 
| Failure: | 0, call _WinAPI_GetLastError() to get extended error information | 
GetDlgCtrlID accepts child window handles as well as handles of controls in dialog boxes.
An application sets the identifier for a child window when it creates the window by assigning the identifier value to the hmenu parameter when calling the CreateWindow or CreateWindowEx function.
Although GetDlgCtrlID may return a value if $hWnd identifies a top-level window, top-level windows cannot have identifiers and such a return value is never valid.
Search GetDlgCtrlID in MSDN Library.
#include <MsgBoxConstants.au3>
#include <WinAPIDlg.au3>
Example()
Func Example()
    Local $idButton
    GUICreate("test")
    $idButton = GUICtrlCreateButton("testing", 0, 0)
    MsgBox($MB_SYSTEMMODAL, "ID", "Dialog Control ID: " & _WinAPI_GetDlgCtrlID(GUICtrlGetHandle($idButton)))
EndFunc   ;==>Example