Remove the Oil filter, cut it open, and look for metal. If metal, check with a magnet. Bearing material is non-magnetic.
Edit: More here: http://www.clubwrx.net/forums/gener...ing-accidents/134587409-03-wrx-rod-knock.html
Edit: More here: http://www.clubwrx.net/forums/gener...ing-accidents/134587409-03-wrx-rod-knock.html