First, as pretty much everyone is saying, validate the XML if there’s a schema defined for it. (If there’s not, define one.)
But you can build tests that are a lot more granular than that by executing XPath queries against the document, e.g.:
string xml="Your xml string here" ;
XmlDocument doc = new XmlDocument();
doc.LoadXml(xml);
path = "/doc/element1[@id='key1']/element2[. = 'value2']";
Assert.IsTrue(doc.SelectSingleNode(path) != null);
This lets you test not only whether or not your document is semantically valid, but whether or not the method producing it is populating it with the values that you expect.