How do i ensure facebook offline access session key is still valid

Question

I can get the offline access session key from PHP API

but how do i know whether its still valid or not before i log him automatically?

require_once 'config.php';  

$facebook->api_client->user = "1294711371";
$facebook->api_client->session_key = "827446297c301f2cc6b581ce-1294711371";

is there any function available?