Still seeking to validate a Document Type Definition

I do not want to validate an XML file per se.  What I want to do is validate a Document Type Definition, in and of itself, to check for wrong syntax, missing internal entities, and so on.  I’m having trouble with validating an XML file, and I think a faulty Document Type Definition is to blame.

The W3C validator is not helping at all, as it keeps sending me to the Nu HTML Checker.