From: Matthew 7:15-20
(Jesus said to His disciples,)  "Beware of false prophets, who come to you in sheep's clothing but inwardly are ravenous wolves.  You will know them by their fruits. Are grapes gathered from thorns, or figs from thistles?  So, every sound tree bears good fruit, but the bad tree bears evil fruit.  A sound tree cannot bear evil fruit, nor can a bad tree bear good fruit.  Every tree that does not bear good fruit is cut down and thrown into the fire.  Thus you will know them by their fruits."
15-20. There are many references in the Old Testament to false prophets; perhaps the best-known passage is Jeremiah 23:9-40 which condemns the impiety of those prophets who "prophesied by Baal and led my people Israel astray"; "who prophesy to you, filling you with vain hopes; they speak visions of their own minds, not from the mouth of the Lord [...]. I did not send the prophets, yet they ran. I did not speak to them, yet they prophesied"; they "lead my people astray by their lies and their recklessness, when I did not send them or charge them; so that they do not profit this people at all."
In the life of the Church the Fathers see these false prophets, as of whom Jesus speaks, in heretics, who apparently are pious and reformist but who in fact do not have Christ's sentiments (cf. St Jerome, "Comm. in Matth.", 7). St John Chrysostom applies this teaching to anyone who appears to be virtuous but in fact is not, and thereby misleads others.
How are false prophets and genuine prophets to be distinguished? By the fruit they produce. Human nobility and divine inspiration combine to give the things of God a savor of their own. A person who truly speaks the things of God sows faith, hope, charity, peace and understanding; whereas a false prophet in the Church of God, in his preaching and behavior, sows division, hatred, resentment, pride and sensuality (cf. Gal 5:16-25). However, the main characteristic of a false prophet is that he separates the people of God from the Magisterium of the Church, through which Christ's teaching is declared to the world. Our Lord also indicates that these deceivers are destined to eternal perdition.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.