function ComPortAvailable(Port: PChar) : boolean;
Var
   DeviceName: Array[0..80] of Char;
   ComFile: THandle;
begin
   StrPCopy(DeviceName, Port);

   ComFile := CreateFile(DeviceName, GENERIC_READ or GENERIC_WRITE, 0, Nil,
                                      OPEN_EXISTING,
                                      FILE_ATTRIBUTE_NORMAL, 0);

   Result := ComFile <> INVALID_HANDLE_VALUE;
   CloseHandle(ComFile);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  if ComPortAvailable('COM1:') then
    ShowMessage('Port available')
  else
    ShowMessage('Port not available');
end;