Well, that's simple and clear as follows:
1. We can know that we are child of God for the Bible tells us. Those who have eternal life are those who believe in Jesus Christ. Those who believe in Jesus Christ are the children of God. ( I John 5: 11-15)
2.. We are given the Holy Spirit at the moment we believe in Jesus Christ. That indwelling Spirit of God affirms us that we are the child of God. " And you also were included in Christ when you heard the word of truth, the gospel of salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God's possession- to the praise of his glory" ( Eph. 1: 13)
3. We experience unspeakable peace that the world can't give , " Peace I leave with you; my peace i give you. I do not give to you as the world gives. Do not let your hearts be troubled and do not be afraid" ( John 14: 27).
4. We experience joy in the midst of sufferings and problems in life, " You have filled my heart with greater joy than when their grain and new wined abound" ( Ps 4: 7). " Though you have not seen him, you love him; and even though you do not see him now, you believe in him and are filled with an inexpressible and glorious joy, for you are receiving the goal of your faith, the salvation of your souls" ( I peter 1: 8).
5. We experience transformation in our life. This transformation is clearly seen in the life of Paul ( Acts 9). We love the Bible, spend time with God and fellow believers. We leave our sinful life and bad habits. We serve our Savior with pure and rejoice in heart.
6. We love Jesus Christ and other people including those who hate us. Loving our enemies can only be seen in the live of believers who experience the forgiveness of God. We put Jesus in the center of our lives and follow His footsteps against bad habits and immorality. Then, We have love and compassion for others, " Oh, that my head were a spring of water and my eyes a fountain of tears! I would weep day and night for the slain of my people" ( Jer. 9: 1). There are still so many points that we can come up with the answers, however, let me leave a chance to answer by others .
Have you experience such kind of life that we mention above ? The child of God experiences those issues what others ( unbelievers) do not understand in life...... shalom!