As finite humans, we do not know a persons heart. Only GOD can see that. However, Jesus did say by their fruits you will know them. All of us struggle with sin, even true christians can struggle with it for a season. But if you truly are saved, the LORD will deal with you. Sometimes the LORD will even take you home!
Adrian Rogers had an analogy in one of his messages about this. He said it would be like a parent taking a child to a birthday party. And while at the party, the child gets into all kinds of trouble by being mean to other kids, destroying the birthday cake, unwrapping the presents, etc. Of course the parent warned the child several times during all of this and even spanked the child a few times. Despite this, the child kept misbehaving. Finally the parent is so embaressed, the parent tells the child, "Come on, I'm taking you home! Your not going to stay around here and ruin my good name!"
GOD said I am holy, therefore you be holy! There is a world full of unbelievers watching.