The method getVersion() of javax.xml.stream.XMLStreamReader is defined to return null if a version wasn't declared. See JavaDoc: http://java.sun.com/javase/6/docs/api/javax/xml/stream/XMLStreamReader.html#getVersion() . The current implementation in JDK 6u7 returns "1.0" by default and never null, even for invalid XML documents.

Example program:
import java.io.ByteArrayInputStream;

import javax.xml.stream.FactoryConfigurationError;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamReader;


public class XMLStreamReaderProblem {
  public static void main(String[] args) {
    try {
      String xmlText = args.length > 0 ? args[0] : "foo bar";
      XMLStreamReader r = XMLInputFactory.newInstance()
        .createXMLStreamReader(newByteArrayInputStream(xmlText.getBytes()));
      String version = r.getVersion();
      System.out.println("Version for text \"" + xmlText + "\": " + version);
    } catch (XMLStreamException e) {
      e.printStackTrace();
    } catch (FactoryConfigurationError e) {
      e.printStackTrace();
    }
  }
}