You need to be a lot more specific than that if you want a useful answer. What exactly do you mean by checking the existence of xml?
There's loads of possible meanings to that e.g: testing for the XML header, testing whether the document is xml, whether the document contains embedded xml, whether the browser can handle xml, whether the javascript engine can handle xml literals, whether an AJAX object is XML etc.
So, ask a more specific question and you'll get better answers.
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