How do we know if we really know God?

In 1 John 4:8 (NKJV) it says, "He who does not love does not know God, for God is love."  So our Heavenly Father is sharing with us via the Holy Spirit, the spirit of truth, that if we do not exhibit love in our life toward God and our fellow man, that there is something missing in our relationship with God.  In order to have this love, we first have to receive this manifested love toward us in the Heavenly Father's sending of his Son (Jesus Christ) to redeem us from sin.  Once we have received the Lord Jesus into our heart and allowed his word to transform us, then it is only natural that we would walk in the knowledge of God by exhibiting this love toward God and our fellow man.  (1 John 4:8-11)