1 John 3:10
 By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother. (ESV)
Many times we have heard people say, "Well, only God knows if that person is a Christian. Only He can judge the heart." In the grand sense of God knowing all things I can certainly agree that God knows those who are his and those who are not. However, here in His Word He has given us a means by which we can discern who belongs to Him and who does not. In fact, the Apostle John says, "It is evident."
So, if it is evident, what is the evidence? First of all, John claims that those who do not practice righteousness are not of God. Notice, that he uses the word "practice." The fact is, all men do some things right some of the time. A brutal dictator may care for his own children. Most people obey the laws of the highway. And as many have often boasted, "Well, I've never killed anyone." But John is talking about practicing righteousness. In other words, living a life of habitual righteousness; finding our identity in righteousness. Those who do not belong to God cannot do this.
 And you were dead in the trespasses and sins  in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience— (ESV)
Paul makes clear that the person who is not born of God is actually following the prince of the power of the air. The spirit that is at work in them is the Devil himself. They cannot practice righteousness because they are in bondage to sin, and they practice sin, or unrighteousness.
The other test that John gives as to whether or not someone is born of God is whether or not they love their brother. I dare say that many people who claim to be born of God are failing this test. Churches that are coming apart because of infighting, grudges, and spiteful conflict. You can be sure that some of those who claim to know God will not hear Him say that He knows them. How tragic.
If you were on trial for your Christian faith would there be enough evidence to convict you? Would your practice of righteousness and your love for your brother be the deciding factor that proved that you were born of God?