Adam Clarke on Matthew 7:16

Ye shall know them by their fruits - Fruits, in the Scripture and Jewish phraseology, are taken for works of any kind. "A man's works," says one, "are the tongue of his heart, and tell honestly whether he is inwardly corrupt or pure." By these works you may distinguish (επιγνωσεσθε) these ravenous wolves from true pastors. The judgment formed of a man by his general conduct is a safe one: if the judgment be not favorable to the person, that is his fault, as you have your opinion of him from his works, i.e. the confession of his own heart.