{ 
 This tip uses the API of Windows to determine 
 if a this available certain COM. 
 Port: COM1, COM2, COM3... 
}

 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;