In the context of the chapter we are commanded to, first, clean our own lives up to be pleasing in the sight of God and to, then, judge righteously. In verses 15 through 20 of this chapter we learn the target of the Righteous Man's judgment, what does the tree or the man bare in the way of fruit? As the followers of Jesus, the Christ, we do not have the power nor the right to judge a fellow man's salvation but if we do not judge the fruit of a man, to determine his or her character we are disobedient servants and worse yet, Worldly Fools.
If we take our children to a baby sitter we do not know ad we have not run a Criminal Background Check on we are setting our children up to be abused in manors we would never dream of. Additionally, a good friend of mine and I often greet one another with Honest Bill Taylor, Used Car Salesman. With us this is a standard Joke, flowing both directions but when I was 14 I was trained by a former Air Force Engine Mechanic to work as a mechanic for a Used Car Lot on Jensen Drive in Houston, Texas and every Houstonian will warn you to never shop for an automobile on Jensen Drive, they be crooked as snakes!
You cannot live a successful life without judging the fruits of the people you chose to deal with. Neither can you ever obey the command of the Great Commission, found in the last three verses of the last chapter of Matthew without judgment. Every Sunday Morning you are seated in the middle of a white to harvest field of unrepentant sinners. And you must get to know them to help them and to help them you must judge the fruit of every one of them.
This is a short lesson but it has eternal bearing on every lost man and woman.