I think it depends. If the person is saying something that can be disproven, disprove it. If the person is passing off their opinion or belief as the standard Orthodox opinion (or the only acceptable one), then show how that's not the case. The fact that a person is not yet a member of the Church has little impact on whether or not they are knowledgeable. There are many baptized Orthodox, even those raised from birth as Orthodox Christians, who have a serious lack of understanding about the faith (which is why Metropolitan Isaiah of Denver recently had to send out a letter about how homosexuality is not ok, but we should love homosexuals nonetheless).