How to validate an XML against schema using JAXB?

I am working with XML and JAXB as I am unmarshalling and marshalling the XML into Java objects and vice versa. Now I am trying to validate our XML against our schema(test.xsd). Suppose if any required field is missing in my XML, then I would like to know which field is missing after validating the XML against schema test.xsd.

public void unmarshal(final InputStream is) {
    final XMLInputFactory factory = XMLInputFactory.newInstance();
    final XMLStreamReader reader = factory.createXMLStreamReader(is);

    Object req = unmarshaller.unmarshal(reader);

    // how would I validate here?
}

How would I validate my XML against test.xsd schema. My test.xsd schema path is -

C:\workspace\one\two\three\src\main\java\com\package\serv\ap\versionOne\test.xsd

UPDATE: loading test.xsd as:

Schema schema = factorySchema.newSchema(new File("C:\\workspace\\one\\two\\three\\src\\main\\java\\com\\package\\serv\\ap\\versionOne\\test.xsd"));

You just need to set an instance of javax.xml.validation.Schema on the Unmarshaller before you do the unmarshal. You can specify an implementation of ValidationEventHandler on the Unmarshaller to catch any problems that occur during the unmarshal process.