Jesus Himself said that a good tree bears good fruit and that we would meet false prophets by their fruit. The fruits are visible, you know that the fruits of an apple tree are in sight, in the same way the actions of people, whether good or bad, are in the light of day.
But the Bible not only tells us about the bad actions of false prophets, there are also a series of fruits that tells us that those who live in the Spirit must possess them:
22 But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith,
23 Meekness, temperance: against such there is no law.