I John 5:2-3 We know we love God's children if we love God and obey his commandments. Loving God means keeping his commandments, and really, that is not difficult.
There are many different ways to know for certain that we are indeed children of God. Two of the most basic are to love God and obey His commandments. To love God involves a lot of different things. When you love God, you want to spend time with Him. You look forward to being alone with Him. He stirs up an excitement in you. You want to please Him in whatever you do. You strive to always be faithful to Him. You long to know Him better and you do all you can to learn as much about Him as you can. You do not ignore Him, neglect Him, or bring shame to Him. You are more concerned about His honor and glory than you are your own. Your life is about Him, not you.
If we truly love the Lord we will obey His Word. We won't pick and choose what we want to do. We will take seriously what God says and strive to do it. We will get rid of sin in our lives. We will strive to keep our souls pure by keeping sin out of them. We will strive to live peaceably with others and treat them with kindness, love, mercy and grace. We will let our actions proclaim our faith and tell whoever we can about Jesus and our love for Him. We will be involved in His church and what we can to serve Him there. We will protect the integrity of God's Word and let it govern our lives.
We know our love for God is real when we can love His children in a godly way. We are not driven by selfish desires or motives. We embrace them as our brothers and sisters, regardless of our differences. We do not let anything become an issue between us that keeps us out of fellowship with them. We live graciously with others and let the Lord handle what needs to be handled in their lives. We pray with them and for them without reservation. We treat them as we want them to treat us.