if (instance of . xs:integer) then 'integer' else 
if (instance of . xs:boolean) then 'boolean' else 
....


or, if you don't want to write a big nested set of tests and are running
saxon, 

http://www.saxonica.com/documentation/extensions/functions/typeannotation.html


type-annotation(item as item()) ==> xs:QName

This function takes an item as argument. If the argument is a node, the
function returns the type annotation of the node, as a QName. If the
type is anonymous, this will be a system-generated internal name. If the
argument is an atomic value, the function returns the type label of the
atomic value, again as a QName.

David


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