In Matthew 7:15-20, Jesus warned His disciples to beware of "wolves" in "sheep's clothing," and based upon the context, it appears their disguise will be so effective, we will not be able to recognize them as Satan's prophets. Putting it in modern terms, He wants believers to become "Missourians." Those of us lucky enough to have been born in Missouri seem to have a built-in skepticism when it comes to listening to the "experts" on any matter. That is why Missouri is called the "Show Me State." Jesus wants God's children to observe the "fruit" produced, and then determine if they are authentic prophets of God.
There are two main ways to evaluate a fruit harvest: one is to count the amount of the fruit. I suggest that this is not a very accurate test of a prophet. Preachers who have "itching ears" tend to preach what is politically correct according to their audience (2 Tim. 4:3-4). It seems that people flock (pardon the pun) to churches that make them feel good, or which preach what they want to hear. If we are not careful, we may see the large mega-churches and come to the conclusion that the pastor is "anointed by God." On the other side of the coin, we may view a small church as being spiritual, when the actual reason it is small is because the Holy Spirit has opened the eyes of former members to see biblical error. Quantity does not appear to be a good test.
The other main way of evaluating the fruit of the prophet is to check the quality of his message. Does the preacher present the Gospel of grace, or a false gospel of works? The true Gospel is the power of God unto salvation to all who believe it (Rom. 1:16). The true Gospel presents the fact that even our belief in the Gospel is a work of God in us, and not something we are able to do ourselves (Eph. 2:8-9). The true Gospel teaches that the death, the burial, and the resurrection of Jesus Christ is what we are to believe, and that we are to openly confess that we believe in Him (1 Cor. 15:3-4; Rom. 10:9-10). And while the true Gospel does not convert the majority of people who hear it, nevertheless, those converted produce the fruit of the Spirit in their lives (Matt. 7:14; Gal. 5:22-23).
Another test of the authenticity of the messenger is the maturity of his "sheep." Do they need to be fed the "milk of the Word," or do they mature and require "meat?" (1 Cor. 3:12; Heb. 5:12-14; 1 Pet. 2:2). Are the "sheep" producing other "sheep?" Some should be increasing the "flock" by thirty-fold, some by sixty-fold, and others by a hundred-fold (Matt. 13:23). Of course, there will always be those "sheep" who are afraid to the threats of the world, and do not reproduce (Matt. 13:20-21). And of course, there will always be those "sheep" who get distracted by the things of this life and produce nothing (Matt. 13:22). But the majority of those in the "flock" should be witnessing for Christ, and leading others to the Savior.
It is easy for a preacher to present himself as a man of God, to live his life above reproach, and to speak flowery words, especially if most of them are already written in the Bible. But one needs to ask, "What is the quality of his fruit?" Don't judge; just observe. The quality of the fruit is what counts.