uses
  {...,} CommCtrl;

function ItemAtPos( TabControlHandle: HWND; x, y: Integer ): integer;
var
  HitTestInfo: TTCHitTestInfo;
  HitIndex: integer;
begin
   HitTestInfo.pt.x := x;
   HitTestInfo.pt.y := y;
   HitTestInfo.flags := 0;
   HitIndex := SendMessage( TabControlHandle, TCM_HITTEST, 0, Longint( @HitTestInfo ) );
   Result := HitIndex;
end;

procedure TForm1.PageControl1MouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
begin
   Caption := IntToStr( ItemAtPos( ( Sender as TTabControl ).Handle, x, y ) );
end;