There are 2 levels of XML validation, well formed, and schema level validation.
This is a basic level of checking and ensures that the document obeys the XML standard (i.e. all the tags are matched and closed etc).
This level of validation is always possible to perform, and certain operations can not be performed until the document is well formed (i.e. Formatting XML, and Inferring an XSD).
Schema Level Validation
This ensures that the structure of the XML document matches that described in an external XML Schema (XSD). In order for this level of validation to be performed, XML Studio must be able to associate an XSD file with the XML document. This can be done in 2 ways via the schemaLocation, or noNamespaceSchemaLocation attributes or via the schema to XML mapping rules See Associating an XML Schema with an XML Document.
You can see the level of validation performed on your XML document by examining the validation confirmation dialog.
This well tell you the state of the document, and also they schemas used to validate it. Any errors will be shown in the error window.
You can see the XML Schemas that are associated with your XML Document by looking at the properties window.