Zk178 - 5:27 am on Aug 9, 2011 (gmt 0)

It seems that the X-Purpose header is sent for "Google Instant" requests, not for "Google Instant Pages" requests. There is even an unresolved bug on this submitted in the Chromium project - [code.google.com...]

So it seems that the only way to distinguish prefetch requests is to use Page Visibility API in client-side scripts.