How can I tell if someone has true faith, or not?

Faith is measured by God by what a person believes, but more importantly, how he lives his life. “How would I know that I have faith?” “For I say, through the grace given unto me, to every man that is among you, not to think of himself more highly than he ought to think; but to think soberly, according as God hath dealt to every man the measure of faith” (Romans 12:3, NIV).
People who have faith have it because they faithfully follow God’s Word. “So then faith cometh by hearing, and hearing by the word of God” (Romans 10:17, NIV). To hear the promises of God everyday will surely build your faith. “For I know the thoughts that I think toward you, saith the LORD, thoughts of peace, and not of evil, to give you an expected end” (Jeremiah 29:11, NIV). “And we know that all things work together for good to them that love God, to them who are called according to his purpose” (Romans 8:28, NIV).
People who have faith build their reward by living the way God desires. “But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him” (Hebrews 11:6, NIV).
People who have faith live by faith. Their actions prove their faith. To know what it means to live by faith, you should know what faith means. Romans 1:17 says, “For therein is the righteousness of God revealed from faith to faith: as it is written, ‘The righteous shall live by faith.’” If a person claims to have faith, but is hateful, he does not have faith. “He who says he is in the light, and hates his brother, is in darkness until now. He who loves his brother abides in the light, and there is no cause for stumbling in him. But he who hates his brother is in darkness and walks in darkness, and does not know where he is going, because the darkness has blinded his eyes” (I John 2:9-11, NIV).
If a person “talks big” (brags) about faith, yet doesn’t follow the truth of God’s Word, they are deceiving themselves, and others. “My little children, let us not love in word or in tongue, but in deed and in truth. And by this we know that we are of the truth, and shall assure our hearts before Him” (I John 3:18-19, NIV).
Faith filled followers of Jesus are lovers of God’s people. The faith-less claim to have faith, but then reject other believers to suit their own purposes. “By this we know that we love the children of God, when we love God and keep His commandments. For this is the love of God, that we keep His commandments. And His commandments are not burdensome” (I John 5:2-3, NIV).
Having faith doesn’t imply that we will always live perfectly. Satan still operates in the world, trying to bring Christians down. We will stumble at times. “If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His word is not in us” (I John 1:8-10, NIV). In the end, the difference between a child of God and a child of Satan is that God’s children pick themselves up after they fall and try to live righteously again.