PREFIX  jfn:  <http://jena.hpl.hp.com/ARQ/function#>
PREFIX  rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX     :  <http://example.org/> 

SELECT DISTINCT ?p
{ ?s ?p ?o
  FILTER ( jfn:namespace(?p) = str(rdf:) )
}