To validate an XML document, right click on the document and choose 'Validate' from the right click menu. If it is valid you will get a dialog box saying all went well. If the document has errors, they will be shown in the standard error Ent.
Not all XML parsers validate XML documents, and your chosen parser must be able to validate XML via DTD for the validation to work. Xerces, the parser that is included with Treebeard, does do validation. Schema validation is not yet implemented.