Examine Yourselves
2Cor 13:2-6 I already gave you a warning when I was with you the second time. I now repeat it while absent: On my return I will not spare those who sinned earlier or any of the others, since you are demanding proof that Christ is speaking through me. He is not weak in dealing with you, but is powerful among you. For to be sure, he was crucified in weakness, yet he lives by God’s power. Likewise, we are weak in him, yet by God’s power we will live with him to serve you.
Examine yourselves to see whether you are in the faith; test yourselves. Do you not realize that Christ Jesus is in you-- unless, of course, you fail the test? And I trust that you will discover that we have not failed the test.
What "proof" was Paul speaking of here? I get the impression that he's speaking of a sort Ananias and Sapphira situation we read of in Acts 5. Paul's going to show up and people are going to start dropping dead. In fact in 1Corinthians 11 he spoke of the Corinthians abusing the communion service, "That is why many among you are weak and sick, and a number of you have fallen asleep." (That is, died) 1Cor 11:30
Though sinless perfection is not a measure of a Christian, there is an expectation that those born of God won't chose to live a lifestyle of sin, and thus behavior is a measure of whether one has been born of God. Speaking in a lifestyle sense, John writes, "No one who is born of God will continue to sin, because God’s seed remains in him; he cannot go on sinning, because he has been born of God. This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not a child of God; nor is anyone who does not love his brother." 1John 3:9,10 and Jesus said, "By this all will know that you are My disciples, if you have love for one another." John 13:35 These are the kind of tests Paul was alluding to. How do you measure up?