You might be able to access the history of your visitor's browser using javascript. I'm not sure of the specifics at the moment, but do a few searches. I haven't used much JS in the last year and have lost my instruction book. :-(
If you could fetch the user's history that would be a major privacy issue.
After a little more research, it looks like I had some overly-ambitious views of history.back() and
history.go(). I don't think it can be done...at least that way.
Thinking about it some more, I'd imagine google doesn't want us to know. They want people to use their system. If we knew who was providing us traffic, we could possibly set up more profitable relationships and bypass adwords/adsense completely.