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.
(Deut. 13:1-3 NIV)
Miracles aren't good enough. You see, God confirmed that Moses was speaking for Him by working unthinkably large miracles through Moses. Later prophets, though, had another test applied to them. Their new teaching had to be in lock-step with the previously revealed teachings. God never contradicts Himself.
That means that a miracle-worker is not automatically speaking for God. We need to compare the miracle-worker's teachings to the teachings God has already revealed. Immediately following Moses, you would need to compare the teachings to those revealed through Moses. By the end of the Old Testament, we need to compare the teachings to the entire Old Testament. The task of determining a true prophet or a false prophet became more and more involved as the years went on.
I'm grateful for the people who worked so diligently to apply these tests to prophets year after year. Now we have a book filled with the writings of confirmed, true prophets. Their teachings read as one large story from One Author...and it is. Peter said it this way:
Above all, you must understand that no prophecy of Scripture came about by the prophet's own interpretation of things. For prophecy never had its origin in the human will, but prophets, though human, spoke from God as they were carried along by the Holy Spirit.
(2 Peter 1:20-21 TNIV)