This is actually a mechanical way of listening for it. It involves taking a metal tube, bending one end shut to seal it off from air, and attaching it somewhere on the block with a very thick hard rubber hose(it has to be hard rubber to prevent sound deadening). Then you run the hose through the firewall and t it off into a set of ear protectors. I believe this simple device would be an invaluable tool when tuning cars on the dyno. I could easily tell when knock occured with the det cans where otherwise I wouldnt have heard it above the rest of the noise. Here are some pics of the actual tube, fittings, and hose attached to my block.
http://img.photobucket.com/albums/v2...e/IMG_1969.jpg
http://img.photobucket.com/albums/v2...e/IMG_1968.jpg
http://img.photobucket.com/albums/v2...e/IMG_1962.jpg