Most people say they believe in the Biblical principle of knowing people by their deeds (Mt 7:16), as we know a tree by its fruit. In practice, however, few people believe this principle. For instance, adultery or homosexual activity doesn't seem to disqualify someone from public service. The false reasoning is that private lives of people don't affect their ability to make decisions and relate to others.
Moreover, this principle of knowing a tree by its fruit is subverted by the rejection of objective truth in our secularized society. If truth, good, and evil are whatever we want them to be, then knowing a tree by its fruit is merely judging a tree according to our whims and/or prejudices, irrespective of reality.
Even those who believe that we can know a tree by its fruit sometimes misunderstand what is good fruit. People prophesy, exorcise demons, or do many miracles in Jesus' name (see Mt 7:22). Nevertheless, this fruit is only ultimately good if the people producing this fruit do the will of the heavenly Father (Mt 7:21).
Some trees are covered with "leafy show" (see Mk 11:13), that is, they are pleasant to behold but have no fruit to nourish others. Like the Pharisees of old, some leaders today present a smooth external image. Jesus calls us to be "fruit inspectors" (see Mt 7:20). Knowing a tree by its fruit ultimately means knowing people by their obedience to the Lord's will and denial of their own wills. Can you recognize a good tree? Are you a good tree?