Social Media Expert

How to identify an expert in anything? Maybe judge by results?

How to identify a lame viral marketing attempt?

Yes... I wondered if we might see a followup to his question in which he provides his own answer by pointing out a "Social Media Expert"

But it's kind of a neat question in the general sense? How do people decide who is an expert in anything? Credentials? What if you don't trust the credentials? I think it all boils down to functionality. But then again, I'm a god forsaken atheist, so I would think like that.