So, your question is "out of a large group of people chatting, some possibly lying, detect the expert"? This is a hard problem - kind of the reverse the Turing test. I could imagine this being a ph. D. research problem. None the less, I'd also expect a great deal of research has already be done (and published) about this problem. So you might want to take a trip to the university library.

Personally, I'd solve the problem in a totally different way. When I arrange the details for the expert, like when and where to log in, I'd give him a user-id/handle/nick/whatever. Then I know who the expert is before hand.

Abigail