Albert Barnes on Matthew 7:16

Ye shall know them by their fruits. He gives the proper test of their character. Men do not judge of a tree by its leaves, or bark, or flowers, but by the fruit which it bears. The flowers may be handsome and fragrant; the foliage thick and green; but these are merely ornamental. It is the fruit that is of chief service to man; and he forms his opinion of the nature and value of the tree by their fruit. So of pretensions to religion. The profession may be fair; but the conduct--the fruit in the eye of the world--is to determine the nature of the principles.

(e) "by their fruits" Mt 12:33