How to determine if a string value is in XML format?

I'm connecting to an API, or service like that, which returns me an xml when it finds data in the database, but unfortunately it returns a string if nothing is found or an error occurs. In my application I'm using the following code to connect to this API and retrieve the data:

 WebRequest request = WebRequest.Create(_fileUrl);

 HttpWebResponse response = (HttpWebResponse) request.GetResponse();

 Stream dataStream = response.GetResponseStream();

 var reader = new StreamReader(dataStream);

 string responseFromServer = reader.ReadToEnd();

 xmlDoc.LoadXml(responseFromServer);

As you might expect when there is no data in the database, an error occurs when giving a load in xmlDoc because the responseFromServer variable is not XML but a string.

The question is: How do I identify whether my string returned an xml or not? I do not want to "" try to give a load and if it does not work I understand that what came up was a string. I may be masking other errors when doing this.