Once you ask for, and are granted, offline access the session key you get in the HTTP POST parameters from Facebook (fb_sig_session_key) will have no expiry.
You can check you have such a key by checking the fb_sig_expires parameter, if this is "0" then the session key has no expiry.
If you have an auth_token then you can call getSession to get the session key and check the expires field:
var auth = _auth.getSession(AuthToken);
string sessionKey = auth.session_key;
long uid = auth.uid;
bool expires = auth.expires > 0;
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