As believers, we know what the truth of the gospel is. We have learned what is error as well. Those who have made the choice to not hear and be obedient to the Word of Truth are spiritually blind. They not only are blind, but they listening to false doctrine.
We are known of God because we are obedient to His commandments. We are able to hear the voice of the Shepherd because we are His sheep. Those who are not of God do not hear His voice. They are being led by that which is not righteous. This is how we know whether a person is following the truth or error.