True enough. That's why we are not to make judgments. Man sees only with the eyes. God sees what is in a man's heart.
Yes. But we also can tell by their fruits. If they show no signs of repentance and continue on in the sin, then we know there was no true and Godly repentance. On the other hand, if they do show signs of repentance, we still cannot know their heart on that matter much of the time. Sometimes though it is obvious and the Spirit will witness to us. But it is wrong to judge their hearts because that belongs to the Lord. We can just know that in the end, we each will stand in front of the Lord God and be accountable for each of our thoughts, words, and deeds. And that should be a most humbling Truth to each.