How other websites detect that you are logged in facebook?

If a user is logged in facebook how will i detect that im my website? how d开发者_如何学Coes iframe takes care of it. I need methods beside iframe.

From the Javascript SDK:

FB.getLoginStatus(function(response){
    if(response.session){
        //user is logged in
    }else{
        //user not logged in
    }
});