XML Validator and XML Schema Validator free online tools are now available on the Liquid XML Editor web site.
Simply copy and paste your XML data into the Validators and click the Validate button to test the data against the W3C standards.
The XML Validator will check you XML document is well-formed, but it will also allow you to add a list of URLs to your XML Schema in order to ensure your XML document valid against the rules they define.
These are online now and are 100% free to use!