Ye shall know them by their fruits - not their doctrines - as many of the elder interpreters and some later ones explain it - for that corresponds to the tree itself; but the practical effect of their teaching, which is the proper fruit of the tree.
Do men gather grapes of thorns - any kind of prickly plant.
or figs of thistles? - a three-pronged variety. The general sense is obvious - Every tree bears its own fruit.