How to tell if a paper is behind a paywall? I am trying to figure out if a paper is open access or not. My university subscribes to many non-open access journals. Some journals offer mixed models where some papers are open access and others are not.

Some journal websites tell me I have accessed them through an institutional subscription, but if the journal uses a mixed model of open access, that does not mean all papers are behind a paywall. Is there a reliable way to tell if a paper is behind a paywall? 

In particular I am interested in knowing if this Nature paper is behind a paywall.
To answer your particular question: When trying to read it from home, it tells me "To read this story in full you will need to login or make a payment (see right)." (costs: 30 $US).

You can easily check this by logging in from home, some public WiFi, or even with your mobile device (as long as it is not using WiFi in your university network to connect).