The great criterion which characterizes true love of God is the love of our neighbor. This is an infallible standard because theological love is unique, and though difficult to test in relation to God, it is easily verifiable in relation to the neighbor.
We do not need much discernment to perceive whether our love of neighbor is a matter of words, or is a concrete reality, proved in deed. St. Teresa of Jesus teaches her daughters:
We cannot know whether or not we love God, although there are strong indications for recognizing that we do love him; but we can know whether we love our neighbor. And be sure that the more advanced you are in love for your neighbor, the more advanced you will be in the love of God. (Interior Castle)
When fraternal love is sincere and heartfelt and shown by deed, there can be no doubt that God's love is perfected in us (1 John 4:12).
Fr. Gabriel of St. Mary Magdalen, OCD