Though disciples are not to be judgmental of others (Matt 7:1-5), Jesus does teach us to know who we are dealing with. The Kingdom of God is precious, and its resources are not to be treated casually or carelessly. Some situations might only be a waste of time, or, worse, putting Kingdom things at risk of being abused and mistreated. Disciples are to have some "street smarts" as we go about our mission for the Lord.
Disciples of Jesus are as prone as anyone to make judgments about others. In this passage Jesus teaches us to pay attention to our judgmentalism. It might be a sign that we have much bigger issues in ourselves than those we call attention to in others.