Whether it's in fashion, or food, or forensic science, someone who really knows his stuff about a topic is a maven, or a person particularly skilled in the field.
The word maven comes from the Yiddish meyvn, meaning "one who understands." But to be a maven you have to more than just understand a topic, you have to know its ins and outs. Often mavens are the people that you turn to as experts in a field. You don't become a maven overnight. That kind of expertise comes with an accumulation of knowledge over the years.