Windows (and other OSes) already do this: http://blog.superuser.com/2011/05/16/windows-7-network-awareness/ http://blog.superuser.com/2011/05/16/windows-7-netwo

Windows (and other OSes) already do this: http://blog.superuser.com/2011/05/16/windows-7-network-awareness/ http://blog.superuser.com/2011/05/16/windows-7-network-aware... Basically, systems looks up a particular domain name and then requests a text file from that server. It then also checks if another domain name resolves to a hard-coded IP. If both checks pass, you're on the internet.