Let's let John Piper respond:
If someone asked me this morning, "How do you know that you were born?" I would not reach for a birth certificate and argue that a doctor signed it on January 11, 1946, in Chattanooga, Tennessee. I would say, I know I was born because I see and hear and feel and I get hungry and I breathe. I am alive!
And so if someone asks you today (or if you ask yourself!), have you been born again, what will you say? How can you know? You know whether you have been born the second time (born of the Spirit) the same way you know whether you were born the first time. Do you see the truth of the beauty of the gospel? Do you hear the voice of God in the gospel? Do you feel the need to repent and be forgiven? Are you hungry for the milk of God's Word (1 Peter 2:2)? Are you breathing the air of grace? Are you alive with hope in the promises of God? A living hope through the resurrection of Jesus Christ from the dead! Amen.