Albert Barnes on 1 John 4:13

Hereby know we that we dwell in him. Here is another, or an additional evidence of it.

Because he hath given us of his Spirit. He has imparted the influences of that Spirit to our souls, producing "love, joy, peace, long-suffering, gentleness, goodness, faith," etc., Gal 5:22,23. It was one of the promises which the Lord Jesus made to his disciples that he would send the Holy Spirit to be with them after he should be withdrawn from them, (Jn 14:16,17,26, Jn 15:26, 16:7;) and one of the clearest evidences which we can have that we are the children of God, is derived from the influences of that Spirit on our hearts. See this sentiment illustrated in the Rom 8:16.

(h) "Hereby" Jn 14:20, 1Jn 3:24