There are many ways to recognize Google sites. Some of them are reading the about page of sites, checking in Wikipedia, searching in forums and so on. But these are the lengthy ways and you know people are in a hurry in the internet. So the shortcut is to log in any one site of Google like Gmail, Blogger, etc. Thereafter open another tab and type the URL of the new site. If it belongs to Google, you are automatically signed in there. Have a fun.
Discovered by +Don Prince