Pray for yourself and the other person. Pray that you would be faithful merely to speak truth, relying on God’s Spirit to do the work of true persuasion (1 Cor. 3:6-7; 2 Cor. 7:8-10). Pray that he would convict and change the heart.
Speak the truth in love (Eph. 4:15). Approach the person tenderly, patiently, and kindly. Explain that you are confronting them out of a deep and loving concern for their eternal good.
Point them to the Scriptures. Explain that your goal is not to render a final verdict on their souls. Rather, you are concerned they aren’t living like Scripture says a Christian lives. Refer them to passages like Matthew 7:13-29, Romans 6:12-23, 8:13, 1 Corinthians 6:9-11, 2 Corinthians 13:5, and the whole book of 1 John.
Question them gently. Ask things like, “Do you think your life matches the Bible’s picture of a genuine Christian? Are you genuinely fighting sin or secretly cherishing it? Do you believe that being a Christian means repenting of your sin and trusting in Christ?”
Remind them of their profession of faith and baptism. Remind them of the gospel.
Urge them to consider eternity. Remind them that their eternal joy or condemnation is at stake (Ps. 49; Matt. 25:31-46).