arcpy.Describe(<relationshipclass>).OriginClassKeys or arcpy.Describe(<relationshipclass>).DestinationClassKeys
[(u'OBJECTID', u'OriginPrimary', u''), (u'ParcelID', u'OriginForeign', u'')] [(u'id', u'DestinationPrimary', u''), (u'RenterID', u'DestinationForeign', u'')]