If you have a reference to the Document, you can loop through all immediate child nodes and look for a Element Node.
Document doc;
...
NodeList children = doc.getChildNodes();
for (int j=0; j<children.getLength(); j++) {
Node n = children.item(j);
if (n.getNodeType() == Node.ELEMENT_NODE) {
System.out.println("Root Node name is: " + n.getNodeName());
}
}