Thus you will recognize them by their fruits. (Matthew 7: 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 recognize them by their fruits. Are grapes gathered from thornbushes, or figs from thistles? So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus you will recognize them by their fruits. (Matthew 7: 15-20)
A prophet is someone who speaks in the name of God. False prophets are, therefore, those who say they are prophets but who actually announce their own will instead of the Will of God. It is easy to distinguish a false prophet from a true one. If the fruits of their preaching are in line with the divine commandments, then the prophet has been called by God. On the other hand, if the fruits of their proclamations contradict God's Will, then it means that this person has not been called by God and sooner or later he will be cut down and thrown into the fire. With hindsight, all will be clear. Jesus only asks that we do His Will by trying to reap the good fruits that His word helps grow.