You gauge a lot of "progress" for Bitcoin by determining whether or not people are actually using it, and one major incident, such as walking to your local grocer and seeing that they are accepting Bitcoin, would be a pretty good indicator of how Bitcoin is doing on the world stage, and how it is being integrated into the rest of the economy. If you start seeing people talking about Bitcoin or paying with Bitcoin very consistently, then you know for a fact that it has become "mainstream". Beyond that there isn't much.