This passage begins with the assumption that the prophet does foretell the future accurately or perform some other, humanly impossible work. Nevertheless, if that prophet's central message is to follow after a different god or to take a spiritual path that the true God has not said to take, that person is a false prophet. God states unequivocally that misrepresenting Him incurs the death penalty, and Revelation 19:20 says that this is exactly what happens to the False Prophet: He is thrown into the Lake of Fire.
The message of the false prophet is contrasted in Deuteronomy 13:3-4 with loving the true God with all of our heart and soul (life), walking after Him, fearing Him, keeping His commandments, obeying His voice, serving Him, and holding fast to Him. These elements indicate what God wants His people to be focused on, helping to define whether a man claiming to speak for God is truly doing so or not.
Verse 4 mentions obeying God's voice and keeping His commandments. This is a regular theme with God's true prophets: They always have God's law undergirding their messages. When the Old Testament prophets were sent to warn or inform Israel and Judah, they always pointed out the grievous ways in which the people had transgressed God's law.
False prophets, on the other hand, will not hold the moral line that God requires. Lamentations 2:14 says that the false prophets "have not uncovered your iniquity, to bring back your captives, but have envisioned for you false prophecies and delusions." False prophets will not connect the dots between the sinfulness of a nation and national calamity. They instead focus on something other than God's standard of righteousness.
This same principle appears in Isaiah 8:19-20. Both houses of Israel were guilty of seeking out mediums and wizards for spiritual guidance, and God's response is very telling:
And when they say to you, "Seek those who are mediums and wizards, who whisper and mutter," should not a people seek their God? Should they seek the dead on behalf of the living? To the law and to the testimony! If they do not speak according to this word, it is because there is no light in them.
God gives us a standard by which to measure the words of a prophet: the law and testimony - His Word. If the prophet's message contradicts what is already established as God's Word, it is evidence that he lacks spiritual understanding. If his words do not line up with God's law and testimony, he is not speaking the truth.
In summary, the hallmark of a true prophet is his upholding of the law of God, while false prophets dodge moral teaching and instead preach a message that appeals to the masses. God's truth - and His law in particular - is abhorrent to the natural mind (Romans 8:7), and thus it is quite common for God's prophets to be killed, while the false prophets enjoy widespread popularity and support.
The current trend of outcome-based churches serves as a good example. Their leaders preach a widely popular message, and thousands of people follow them. Yet, Jesus says in Luke 6:26, "Woe to you when all men speak well of you, for so did their fathers to the false prophets." Popularity is not a good measurement of God's pleasure with a leader!
Jesus Christ, the most perfect Spokesman for God who has ever lived, only had about 120 true followers when His ministry ended (Acts 1:15). This was not due to failure on His part, but because His Father's message could be wholeheartedly believed only by those whose minds God had already prepared to accept it.
"Purpose-Driven" church leaders will not preach the unadulterated Word of God because they know it is divisive. It would also thwart their goals of a large following and a large income. Thus, their messages do not involve repentance, sound doctrine, or God's law, except where it may serve to further whatever purpose is driving them. Their messages do not remind people of their moral responsibilities to God and brother, and thus if they claim to speak for God or say that God sent them, we can know from biblical patterns that they are, in fact, false prophets. Their large churches, as amazing as they might seem, are not accurate indicators of God's involvement or blessing.