Jesus speaks of false prophets elsewhere, and lays out the appropriate test:
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.
A false prophet, therefore, is someone who claims to be a prophet, but whose works are evil, whereas a true prophet will bring forth good fruit, and his works will be edifying and uplifting of those around him, as per the teachings of Jesus and the principles of the Gospel.