There are many ways to tell a good from a bad conference. My personal definition of a good conference is one where you get more than you invested. To go to a conference you invest time, money, perhaps have to prepare a presentation, socialize and spend long evenings into social dinners. Is it worth it?
I believe that you will bring home little if you just go to sessions. What would be the difference between watching the conference via streaming and being there? So, my general rule is that you have to work your way through each day during a conference...