How do I check if the scanner is plugged in (C#, .NET TWAIN)

enum AcquireResult
{
    OK = 0,
    InitFailed = 1,
    DeviceIDFailed = 2,
    CapabilityFailed = 3,
    UserInterfaceError = 4
}
private void StartScan()
{
    if (!_msgFilter)
    {
        _parent.Enabled = false;
        _msgFilter = true;
        Application.AddMessageFilter(this);
    }
    AcquireResult ar = _twain.Acquire();
    if (ar != AcquireResult.OK)
    {
        EndingScan();
        switch (ar)
        {
            case AcquireResult.CapabilityFailed:
                throw new Exception("Scanner capability setup failed");
            case AcquireResult.DeviceIDFailed:
                throw new Exception("Unable to determine device identity");
            case AcquireResult.InitFailed:
                throw new Exception("Scanner initialisation failed");
            case AcquireResult.UserInterfaceError:
                throw new Exception("Error with the Twain user interface");
            default:
                throw new Exception("Document scanning failed");
        }
    }
}

case TwainCommand.Null:
	{
	 EndingScan();
	 tw.CloseSrc();
             Msgbox("There is no device or the scannning has been cancelled.");
	 break;
	}