How does Android measure connectivity state?
All of the information about it is pretty vague. Probably because they don't want people attacking a particular Google server.

For instance, the Samsung Galaxy Tab 10.1 owners manual has this to say:


  Note: Network status icons turn blue if you have a Google Account added to
  your tablet and the tablet is connected to Google services for syncing
  your Gmail, Calendar events, contacts, backing up your settings, and
  so on. If you do not have a Google Account or if, for example, you are
  connected to a Wi-Fi network that is not connected to the Internet, the
  network icons are gray.