Looks like the best you can get is a JS property once the page is loaded. http://code.google.com/chrome/whitepapers/pagevisibility.htm...
Why do you say that? They seem to send "X-Purpose: instant", at least according to their FAQ[1] and bug reports[2].
[1]: http://www.google.co.uk/chrome/intl/en-GB/webmasters-faq.htm...
[2]: https://code.google.com/p/chromium/issues/detail?id=91735
Looks like the best you can get is a JS property once the page is loaded. http://code.google.com/chrome/whitepapers/pagevisibility.htm...