<Label Text="{Binding DeviceGuid}"/>

BindingContext = new BluetoothViewModel();

private string _deviceGuid;
    public string DeviceGuid
    {
        get
        {
            return _deviceGuid;
        }
        set
        {
            Console.WriteLine("Guid: "+_deviceGuid);
            if (_deviceGuid != value)
            {
                Console.WriteLine("geändert Guid: " + value);
                _deviceGuid = value;
                OnPropertyChanged();
            }
        }
    }

ViewModel a = new ViewModel();
a.DeviceGuid = "test";