Detecting facebook logout from iframe application

How can i found in an iframe application whether the 开发者_JAVA百科user is already logged in or not.

I am initially authorizing the user and got the access token and store the fb profile id in Session variable. But i am not able to find out if a user logged out from facebook after that.

Can someone help me to find out how we can do this?

You would use the Facebook Javascript SDK and call FB.getLoginStatus(). Another option would be to call the Facebook graph api with the access token you are storing in session and see if that access token is still valid.