Most people have a hard time with the "knowing someone by their fruits." And truthfully it's a part of a much bigger issue. Knowing what fruit is. Knowing how to bear fruit.

For there is no good tree which produces bad fruit, nor, on the other hand, a bad tree which produces good fruit. For each tree is known by its own fruit. For men do not gather figs from thorns, nor do they pick grapes from a briar bush. The good man out of the good treasure of his heart brings forth what is good; and the evil man out of the evil treasure brings forth what is evil; for his mouth speaks from that which fills his heart.
“Why do you call Me, ‘Lord, Lord,’ and do not do what I say? Everyone who comes to Me and hears My words and acts on them, I will show you whom he is like: he is like a man building a house, who dug deep and laid a foundation on the rock; and when a flood occurred, the torrent burst against that house and could not shake it, because it had been well built. But the one who has heard and has not acted accordingly, is like a man who built a house on the ground without any foundation; and the torrent burst against it and immediately it collapsed, and the ruin of that house was great.”
— Luke 6:43-49