Jesus told them to be "fruit inspectors." He had spoken to them earlier about being judgmental (Mt. 7:1-5); that is, evaluating others in order to find fault with them. And now, He was simply telling them to be on guard against those professing to be believers in order to assume a role of leadership within the assembly.
What kind of "fruit" should one expect to find if the person really possess faith in Christ? Well, the first thing one would need to discover is whether or not the person has a secure "foundation" for his faith (Mt. 7:24-27; 1 Cor. 3:10-11). Jesus, in telling them how to attain everlasting life, said all one truly needed to be saved was to believe on Him. He said, "...This is the work of God, that ye believe on Him (Jesus) whom He (the Father) hath sent" (Jn. 3:16; 6:25-29). Once the person claims that his faith is founded on Christ, it should be easy to discern if he is telling the truth. Remember what Jesus said in verse twenty-one. Those who are true believers will do the will of His Father! When one's life is in harmony with God, he displays the fruit of the Spirit: "...love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, (and) temperance..." (Gal. 5:22-23). However, when a born again believer is walking in the flesh instead of the Spirit, he produces a totally different kind of fruit: "...adultery, fornication, uncleanness, lasciviousness, idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, envyings, murders, drunkenness, revellings, and such like..." (Gal. 5:19-21).
The world, and even many Christians believe that saved people do not ever produce "bad fruit," because they have the crazy notion that becoming a born again believer changes a person into being perfect. Paul and John clearly taught otherwise (Rom. 7:14-25; 1 Jn. 1:5-10). Those same people object to the doctrine of the eternal security of a born again believer, because they fail to recognize it is God who promises to complete what He has begun in His child, even if it takes a sound thrashing (Phil. 1:6; 2:13; Heb. 12:5-11)!
Christians are to be filled with the Spirit, but unfortunately, we all "leak!"