UltraEdit supports the free XMLlint Tool which can parse an XML file for errors. This tool can also be used for DTD validation. I don't have any experience with that tool. Read in help of UltraEdit about XMLlint and use the forum search to find other topics about that tool.
Best regards from an UC/UE/UES for Windows user from Austria
There is also a very good XML validator, written specially for UE, that will not only check your XML for well-formedness, but also validate it against a DTD/XSD schema. You can download it for free at http://www.martinstoeckli.ch/xmlvalidat ... dator.html