This question in the title is a good witnessing tool. Not only good, but this is how Jesus preached and other missionaries like John Wesley did as well.

First, ask someone the question in the title. Most will say yes. Then ask them if they kept the 10 commandments. Most will say "Mostly", some will say "Not so much." But it is vital to go through the commandments, because the Bible says "The law of God is perfect, converting the soul."

1. Have you ever lied? If yes, what does that make you? A liar.
2. Have you ever stolen anything? A lot of the times, they will say no, but then tell them that they already admitted that they are a liar. And if they said, yes, what does that make them? A thief.
3. Have you ever look at a woman with lust? Almost anyone would say yes. Not only men, but women can lust after men as well. Jesus says that whosoever looked upon a woman (or man) with lust has committed adultery in his heart.
4. Have you said God's name in Vain. Like Oh my blank? That is called blasphemy.
That is 4 of the 10 commandments and it says when you kept all of the commandments and broke 1, he is guilty of all.

But I think it is crucial to go through the commandments to point out that he is a sinner and he is in need of a Savior. You are not pointing out the sins, but the Word of God is. You are just asking questions if they kept up with the Law of God. But unless they see that they are guilty before God, they cannot come to the foot of the cross to be saved.