A prophet is generally known as a mouthpiece of God. In other words, they are an intermediary between God and man. Old Testament prophets spoke for God, in the name of God, and carried God's message to others. The authenticity of a prophet is judged by their fruits as Jesus said that one should judge a prophet by his fruits (Matthew 7), and by checking whether his predictions come true.
"If a prophet, or one who foretells by dreams, appears among you and announces to you a miraculous sign or wonder, and if the sign or wonder of which he has spoken takes place, and he says, 'Let us follow other gods' (gods you have not known) 'and let us worship them,' you must not listen to the words of that prophet or dreamer. The LORD your God is testing you to find out whether you love him with all your heart and with all your soul. It is the LORD your God you must follow, and him you must revere. Keep his commands and obey him; serve him and hold fast to him. That prophet or dreamer must be put to death, because he preached rebellion against the LORD your God, who brought you out of Egypt and redeemed you from the land of slavery; he has tried to turn you from the way the LORD your God commanded you to follow. You must purge the evil from among you."
"You may say to yourselves, 'How can we know when a message has not been spoken by the LORD ?' If what a prophet proclaims in the name of the LORD does not take place or come true, that is a message the LORD has not spoken. That prophet has spoken presumptuously. Do not be afraid of him."