[OpenID] How to detect if a user is already logged

[thomas Armstrong]

Hi.

I'm using "PHP OpenID Server by JanRain" as an OpenID server. It works
ok and I can use web tools which require OpenID.

However, I'm creating another webtool and I want it to detect if a
user is already logged in my server without typying their OpenID URL
before.

Is there any protocolo already defined? I know that some web sites use
a JavaScript redirection and a token system.

Regards,
--thomas