Как узнать, форма активна или нет?

type
   //...
  private
    { Private declarations }
    procedure WMNCACTIVATE(var M: TWMNCACTIVATE); message WM_NCACTIVATE;
  end;

implementation

//...

procedure TForm1.WMNCACTIVATE(var M: TWMNCACTIVATE);
begin
  inherited;
  if M.Active then
    caption:='Form active'
  else caption:='Form not active' ;
end;