There was an article a while back about how Googlebot is likely a version of Chrome that fully renders a page to process it. [1] You're right, without a full browser engine, you could pull a whole host of tricks to make the page appear clean but then put in ads later. Since Googlebot is probably a full featured browser, it becomes a lot harder to trick.
[1] http://ipullrank.com/googlebot-is-chrome/