tedster - 12:07 am on Mar 25, 2012 (gmt 0)
In each case: How can g### tell?
Chrome. Toolbar data. Clickstream data from their own free wi-fi. Clickstream data purchased from other ISPs. I'm sure there are more possibilities but these are off the top of my head.
Bing also talked about watching these kinds of user signals - including "does the visitor scroll".