As I understand it, they read two years of his posts and picked up on all the clues.
I think the way they discovered it was that he prepared an amicus brief on behalf of FreeRepublic in its fight with the LA Times and the Washington Post and probably mentioned that fact on one of his posts. They then went the through the legal briefs to find out who filed an amicus brief against them that was an attorney in the South and -- Voila! -- Buckhead is revealed.