There's a piece of software floating around the net that will tell you exactly how many shutter actuations it's done.
Take a tip, set the camera to jpeg small, operation setting to manual, aperture 10ish, Iso 1600, shutter speed 2000/sec
Set the camera to high speed and fire away indescriminantly. If you hear the shutter getting stuck (as opposed to slowing down due to the memory buffer filling up) don't buy.
I got a 40D at 14k genuine clicks and the shutter failed at 17k, this isn't normal for a 40D, usually you get 100k out of them but as it happened to me so soon after purchase do this to rule out any less than savoury reasons for the sale.
Great great camera the 40D, I really rate it.