The XML Schema specification is very complex and to be able to make sure your XML Schema is correct, you should test it using a subset of the following tools.
The Good
Xerces (XML Parser):
Provides full XML Schema validation and validation of XML Documents against an XML Schema.
XSV (XML Schema Validator):
Provides full online XML Schema validation.
Saxon (XSLT 2.0 Processor):
Provides full XML Schema validation.
.NET (XML Parser/XSLT Processor):
Provides full XML Schema validation and validation of XML Documents against an XML Schema.
The Bad
XML Beans (Data Binding):
Supports only a subset of the XML Schema Spec.
JAXB (Data Binding):
Supports only a subset of the XML Schema Spec.
The Ugly
XML Spy (XML IDE):
The XML Schema support does not comply with the spec.
Castor (XML Data Binding)):
The interpretation of the XML Schema definition for complex cases is wrong.