I've been told by one believer that the true prophets were shown to be so because their prophesies came true, but I'm sure that all the so called prophets would have been right some of the time and probably wrong a lot of the time and it would be easy to credit just one prophet (at a time) with all the correct prophecies and not record their mistakes.

It's like a psychic who claims to be talking to dead relatives, the audience is very good at remembering the few positive hits the psychic plucks out of thin air and forgetting that most of his statements were wild generalisations that he twisted to fit when someone started volunteering information. I feel it is the same with the prophets talking to God.

God does threaten to destroy the people of Israel time and time again, but as religion is often about controlling people, this would've be a way for the leaders to keep control by pointing at various disasters that have befallen them and threatening more if they keep up their bad behaviour. So then future disasters are blamed on the people not satisfying God but many survive each famine, drought, or war because these events aren't actually as terrible as the threats made them out to be. Also, in that part of the world you're probably going to be right most of the time that a famine or war is coming up soon, which takes us back to the truth of the prophets.