Matthew 7:15-20

You will know them by their fruit.
v.15 Beware of false prophets by maintaining a healthy balance between not judging (v.1-5) and not being gullible (v.6). They come pretending to have authentic Christianity to conceal their identity/hostility. They do not, however, teach the 'narrow way' that leads to life and therefore are leading those who would listen to them to destruction.
v.16a How do we identify these false prophets? You will recognize them by their fruits. There will be many false prophets (Mt 24:11) and they will lead many astray (Mt 24:24).
v.16b-19 This is now illustrated by the image of good trees producing good fruit and bad trees producing bad fruit. A good tree can't produce bad fruit and a bad tree can't produce good fruit. False prophets will be judged (as a bad tree thrown into the fire, see Mt 3:10).
v.20 Thus, you will know them (false prophets) by their fruit. At the grocery store we inspect the fruit to discern the good from the bad. The fruit of a person is all one says and does. This fruit will eventually give evidence of the character of the person.

Read Matthew 7:15-20