No, I don't; God alone knows a person's heart, and only He knows if a person is sincere in their commitment to Christ or if they are only pretending to be His follower.
This is what Jesus meant when He warned His disciples, "Do not judge, or you too will be judged" (Matthew 7:1). Only God has the right to judge us concerning our eternal salvation, and ultimately we are accountable to Him alone. The Bible says, "There is only one Lawgiver and Judge, the one who is able to save and destroy. But you who are you to judge your neighbor?" (James 4:12).
This doesn't mean, of course, that we are to be naive or undiscerning toward people who may be out to harm us. The Bible tells us to avoid evil people and in order to do that, we have to exercise good judgment. The Bible says, "The wise in heart are called discerning" (Proverbs 16:21). But this is different from judging their relationship with God.
At the same time, don't fall into the same trap this person did; in other words, don't unconsciously let yourself become this person's judge. Instead, ask God to give you an opportunity to share your concern with them. The Bible tells us to speak "only what is helpful for building others up according to their needs, that it may benefit those who listen" (Ephesians 4:29).