procedure TForm1.FormCreate(Sender: TObject);
begin
  with StatusBar1 do
  begin
    SimplePanel:= true;
    SimpleText:= 'Вставка';
  end;
end;

procedure TForm1.RichEdit1KeyDown(Sender: TObject; var Key: Word;
  Shift: TShiftState);
begin
  if (GetKeyState(VK_INSERT) and 1) = 1 then
    StatusBar1.SimpleText:= 'Замена' else StatusBar1.SimpleText:= 'Вставка'
end;

var
           InsertMode: Boolean;
begin
           InsertMode := GetKeyState(VK_INSERT) = 1;