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.
the Java SDK comes with a standard tool called xjc . This tool generates the classes parsing your schema. You could use this code to validate your partners' input.
See also : http://www.java2s.com/Tutorial/Java/0440__XML/ThexjcTool.htm