Can you tell if someone is TRULY a Christian?

It will be interesting when you get into heaven thinking one of your close friends is a christian and he is NOT there. You can't tell who is saved because ONLY God can do that. “O LORD, You have searched me and known me. You know when I sit down and when I rise up; You do scrutinize my path and my lying down, and are intimately acquainted with all my ways” (Psalm 139:1-3). God knows all our habits, plans, aims, desires, attitudes, ambitions, etc. are known to Him. “You understand my thoughts from afar” (v.2b). “He knows the secrets of the heart” (Ps. 44:21b). Jesus said, “God knows your hearts” (Lk. 16:15). The LORD said to Samuel, “God sees not as man sees, for man looks at the outward appearance, but the LORD looks at the heart” (I. Sam. 16:7). Again to King Solomon God said, “The LORD searches all hearts, and understands every intent of the thoughts. If you seek Him, He will let you find Him; but if you forsake Him, He will reject you forever” (1 Chron. 28:9). “He who searches the hearts knows what the mind of the Spirit is, because He intercedes for the saints according to the will of God” (Rom. 8:27). We can "observe" their fruits and see how they behave. Many have accepted Christ by watching Christians more than talking. In 1 Corinthians 13:5 says "Love does not act unbecomingly"; which means a Christian should demonstrate godly love and have real credibility. Your credibility is evangelism.