title: How should I test if an object is a XML document (in a cross browser way)
body: <p>For an unit test, I want to be able to check if a certain returned object is a XML document. What is the best way to do so?</p>
  
  <p>I am currently just testing for <code>doc.implementation</code> (the first DOM property that came to mind) but is there a better way? Also, is there a nice way to tell apart XML documents from HTML documents?</p>