Google only knows how long people stay on the page using Google Analytics and many sites (mine, for instance) don't use it.
I think they also know if there are any ads hosted by google, or if the user is using chrome.
Depending on how moral they are, they could figure it out if the user is on android at all, or if the site has recaptcha, or if they're using google DNS, or if they have a partnership with the ISP, or if the site is loading fonts or javascript libraries from google servers, and probably some others that I can't think of off the top of my head.
I think they also know if there are any ads hosted by google, or if the user is using chrome.
Depending on how moral they are, they could figure it out if the user is on android at all, or if the site has recaptcha, or if they're using google DNS, or if they have a partnership with the ISP, or if the site is loading fonts or javascript libraries from google servers, and probably some others that I can't think of off the top of my head.