"Word-of-mouth traffic can be some of the best, most qualified visitors to your site," says Joy Brazelle. "But, how do you actually know if you are getting traffic from word-of-mouth?"
Brazelle says to dig deeper into your direct traffic stats, particularly branded keywords and non-search referrals. For example, direct site visitors either know your URL and type it directly into their browser, or have your site bookmarked. "As your brand becomes part of more and more conversations, the number of visitors who will know your URL will increase," she says.
You should also monitor your branded keyword stats, as visitors who can't remember your URL--but do remember your name--are likely visitors that have heard about your company through offline channels, including word-of-mouth.