A.T. Robertson Commentary Matthew 7:16

"By their fruits ye shall know them. Do [men] gather grapes of thorns, or figs of thistles?" — Matthew 7:16 (ASV)

By their fruits ye shall know them (απο των καρπων αυτων επιγνωσεσθε). From their fruits you will recognize them." The verb "know " (γινωσκω) has επ added, fully know. The illustrations from the trees and vines have many parallels in ancient writers.