Member Login

Maybe this is already posted somewhere, when I search I only find partial answers…

But, is there a way perhaps thru script or plugin for a WP blog to look at a visitor to tell if the are already logged into Facebook, LinkedIn or Gmail, and if they are, to allow them to view the blog as a subscriber by default. I can get them to register directly later, but to get say access to certain pages as a subscriber.

As I am new to this, I thank you in advance.

dw