A better signal would be looking at the placement of Adsense on the page.
You took the words right out of my mouth Alyssa. A person can almost always take one look at a page and determine if it was built for Adsense. At the one and only website where I use it, it's tucked at the very bottom of some pages ~ which is why no doubt I only see about $100 a year. But given Skweb's posting above, I may need to move it to a considerably more prominent position ~ I'm obviously missing out.