This is a restatement of the New Testament principle of the fruit of the tree.
Matthew 7:15-20
15 ¶ Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves.
16 Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?
17 Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.
18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.
19 Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.
20 Wherefore by their fruits ye shall know them.
The problem of what to believe is always with us. We always have a choice not to believe, and frequently that choice not to believe is much easier than believing. How do we know? The only sure measure the Lord has given us is the test of the harvest: “every good tree bringeth froth good fruit” (Matthew 7:17): “whatsoever thing persuadeth men to do good is of me.” (Ether 4:12).