XPath check for non-existing node

Im having a bit of trouble finding the right XPath syntax to check if a particular node in my XML exists. I'm only allowed to use XPath (so no XSL or something else like that, it has to be a pure XPath expression syntax).

I have an XML and it has a node Filename but it doesn't exist in every case. When the filename isn't specified, my LiveCycle proces will use a different route to fill in the filename. But how do I check if the Filename node exists?