A.T. Robertson Commentary 1 John 5:2

"Hereby we know that we love the children of God, when we love God and do his commandments." — 1 John 5:2 (ASV)

Hereby (εν τουτω). John's usual phrase for the test of the sincerity of our love. "The love of God and the love of the brethren do in fact include each the other" (Westcott). Each is a test of the other. So put 3:14 with 5:2.

When (οταν). "Whenever" indefinite temporal clause with οταν and the present active subjunctive (the same form αγαπωμεν as the indicative with οτ (that) just before, "whenever we keep on loving God."

And do (κα ποιωμεν) "and whenever we keep on doing (present active subjunctive of ποιεω) his commandments." See 1:6 for "doing the truth."