As for you, see that what you have heard from the beginning remains in you. If it does, you also will remain in the Son and in the Father. And this is what he promised us—eternal life.… If you know that he is righteous, you know that everyone who does what is right has been born of him. —1 John 2:24-25, 29
According to John, one of the ways that you can tell if you’re born again is that you are living “right.” Does that mean “perfect”? No, it means that you have a perfect Spirit, and IF you choose to be led by the Spirit of Jesus in your spirit, the rest of your life will be catching up to that. (Fancy word for that is “sanctification.”) Yes, you still live in a fallen world and there will be days when you make poor choices and walk in the flesh rather than in the Spirit. But if you are born again, and you are trusting Christ to live His life through you, His righteousness will characterize your life! That kind of “living right” becomes your heartbeat, and with it comes a humble assurance that, yes, you have been born again and Someone is at work in you and through you to God’s good pleasure.