Fisherman,
Their prophecies and their credentials. Although some prophecies were future to those they spoke to, Bible states that God directly or through the prophets communicated with people proving that God was communicating.
Just because the Bible states that God spoke directly or through the prophets, is no reason to beleive that God "really" spoke through them. The Bible is so full of internal contridictions, that its claims of inspiration can hardly be taken seriously.