The eXtensible Markup Language (XML) is the defacto industry standard for exchanging data on the Web and elsewhere. While the relational model of data enjoys a well-accepted definition of a key, several competing notions of keys exist in XML. These have complementary properties and therefore serve different applications domains. In a nutshell, XML keys allow us to capture important domain semantics in XML documents and thereby advance data processing in most applications. In this paper we propose how to validate XML documents against an expressive class of XML keys using XML Schema and XQuery, respectively. It is somewhat surprising how simple it is to express sophisticated notions of XML keys in these off-the-shelf tools. Experiments show that our simple validation technique works well for real-world data of reasonable size. For large-scale data, however, dedicated tools must be developed.