2:3 Now by this we know that we know Him, if we keep His commandments.
5 But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him.
3:16 By this we know love, because He laid down His life for us. And we also ought to lay down our lives for the brethren.
19 And by this we know that we are of the truth, and shall assure our hearts before Him.
24 Now he who keeps His commandments abides in Him, and He in him. And by this we know that He abides in us, by the Spirit whom He has given us.
5:2 By this we know that we love the children of God, when we love God and keep His commandments.
-1 John 2:3, 5, 3:16, 19, 24, 5:2