I don't think that there's any objective set of criteria for being a folk music critic---the value of the criticism depends upon the tastes of the person reading the review much more than it does to the skill and background of the reviewer. Consider the recent Springsteen tribute to Pete Seeger. It's been praised highly by several (I suppose) knowledgable people; My review (and I suspect I'm somewhat knowledgable) would be on the order of "Amateurish. Awful. Misses the point."
    People who have spoken to me in the past can take my set of biases into account, as they can, say, Ron Olesko's which differ markedly. Either can provide guidance to a prospective purchaser, but only if that prospective purchaser is familiar with mine (or Ron's) past critiques.