If you're a Firefox user, I have a Greasemonkey script available that highlights all the AdSense ad and link units on a page by putting a big red border around them. The ironic thing is that I didn't really write the script, I just tweaked a script someone else wrote to hide those ads! Anyhow, if you're interested, you can get it from here and I have some screenshots showing how it works here. (Greasemonkey is a Firefox extension that lets you run scripts to change the look and behavior of some or all pages shown by your browser. Cool stuff.)
Ok, this thread is really old, but do you guys know if there is something similar available today? The old links do not work any more. Thanks, Cibsi