How does one test a file to see if it’s a valid XML file before loading it with XDocument.Load()?

It’s probably just worth catching the specific exception if you want to show a message to the user:

 try
 {
   XDocument xd1 = new XDocument();
   xd1 = XDocument.Load(myfile);
 }
 catch (XmlException exception)
 {
     ShowMessage("Your XML was probably bad...");
 }