False prophets are known by their fruit.

Matthew 7:15–20

¹⁵Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves.

¹⁶Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?

¹⁷Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.

¹⁸A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.

¹⁹Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.

²⁰Wherefore by their fruits ye shall know them.