At 10.2.1 release, it will be possible to obtain the origin and destination class keys via python using the Describe function.You can use:arcpy.Describe(<relationshipclass>).OriginClassKeys or
arcpy.Describe(<relationshipclass>).DestinationClassKeys
The result returned will look something like this:[(u'OBJECTID', u'OriginPrimary', u''), (u'ParcelID', u'OriginForeign', u'')]
[(u'id', u'DestinationPrimary', u''), (u'RenterID', u'DestinationForeign', u'')]
Hope this helps!