In addition they have Google Analytics on a huge % of pages, so they can track referrals from Bing to those pages, and if the referrals to Google's homepage ecl

In addition they have Google Analytics on a huge % of pages, so they can track referrals from Bing to those pages, and if the referrals to Google's homepage eclipse those, that's a strong indication. ...assuming Bing isn't preventing that behaviour.