title: detecting if user is logged in at different domain
body: <p>I'm developing a rails app that creates a widget that you can put on your website. When the widget is displayed (on a website with a different host) to a user who is logged in my app, I would like to display him some additional admin options.</p>
  
  <p>What would be the best and easiest way to figure out if the user is logged in the app? </p>
  
  <p>I was thinking of storing the IP when user logs in, and then compare the IP from the request that is sent to the widget controller.</p>