Validate an XSD Schema?

The W3C has a online validator for XML Schemas at http://www.w3.org/2001/03/webdata/xsv. Since W3C is the source of the XML Schema spec, their validator should be trustworthy.

If this is a short-term thing, you could use an evaluation copy of a tool like Stylus Studio.

If it's long-term maintenance, you might want to consider purchasing an XML schema editor like Stylus, or Oxygen or Altova.

You didn't specify the source language, but it's only a few lines of code to write a schema validator in Java or .Net.