In general, I advocate empiricism. In the context of making long-term health-related decisions (like what sort of diet to follow), this means hunkering down and reading research and research reviews, and then coming to your own conclusions.
But I must admit, that takes a crap load of time. Given the exponentially rising amount of information, along with the amount of bad science out there, sometimes you'll just have to find trusted bodies to help you to filter out information.
When you have to resort to that, always pick the people who only ever use the research, and not their qualifications, to back up their claims.