Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves.
 Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?
 Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.
 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.
 Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.
 Wherefore by their fruits ye shall know them.— KJV, Matthew 7:15-20
A false prophet is someone that has bad fruit.
Love worketh no ill to his neighbour: therefore love is the fulfilling of the law.— KJV, Romans 13:10
A false prophet has bad fruit, that is he or she worketh evil to his neighbour.
Notice that it says wolf in sheep clothing . So start looking for the sheep clothing to find the wolf. That basically means that you are likely to find them in churches. This is why I advice people to stay away from organized religion.
How should we deal with a false prophet ?
Deal with them as with the evil man. That is depart from them and avoid them.