How should we judge a witnessed miracle's source?

Les explains that Paul asks the Galatians whether the Spirit and miracles among them came by the works of the Law or by the hearing of faith. Under the working of the Law, they could not perform any miracles; it was impossible. But when the Holy Spirit activated the hearts and minds of those pagan Galatians and others throughout the Roman Empire, it produced a miraculous transformation from paganism, idolatry, and immorality into a whole new lifestyle. Les notes that is a miracle in itself, and though other manifestations of the miraculous existed in the early church, Paul does not explicitly delineate them; the question remains the same. If a miracle was witnessed, it must be judged as either through keeping the Law or through the manifestation of the Spirit activated by faith. The theme of Galatians is that we are not under Law but under grace, and we come under grace and out from under the Law by faith plus nothing. Les warns not to be suckered into a works religion but to maintain salvation by grace through faith, plus nothing.