title: How do some web services recognize I'm logging in from a new location?
body: <p>I've noticed that several web services (namely Steam, Facebook, Google, etc) are able to detect a new browser/new location, and on some cases, require me to validate it with an emailed code.</p>
  
  <p>How do they do that? Do they use a cookie? Some other way? How reliable is that?</p>
  
  <p><strong>How can I detect if a user is logging in from his "normal" environment, be it same browser, same computer, same approximate location?</strong></p>