As I've been having to talk about "authenticity" a lot recently, as have many people in the world of brands and communications, I've had to think about what makes something authentic. For me it came down to two things: Credibility and accountability. Obviously it has to be something that is believable but also you should be able to put your name, reputation, brand status on the line for something if it is authentic.
That's why expertise (as discussed here by Russell Davies) can serve to support a claim to authenticity. Because if your tips don't work, or your advice doesn't help or your products don't do what they are supposed to do, then you can't be an expert and you are therefore not authentic. However if in claiming to be an expert you throw your hat in the ring and say OK guys, I know how to do this and it works and you are right, then you are a genuine, credible and authentic expert.
http://www.youtube.com/watch?v=QbCoLN1WgiM
Can't seem to link directly to YouTube as they don't support Typepad but maybe that's just my crapness