Matthew 7 : 15-20

Beware of false prophets : they come to you in sheep’s clothing, but inside they are wild wolves. You will recognise them by their fruits. Do you ever pick grapes from thornbushes, or figs from thistles? A good tree always produces good fruit, a rotten tree produces bad fruit. A good tree cannot produce bad fruit, and a rotten tree cannot bear good fruit.

Any tree that does not bear good fruit is cut down and thrown in the fire. So you will know them by their fruit.