Jeremiah ch.28 - How to Identify a False Prophet!

In this chapter Jeremiah faces down Hananya Ben Azur. But as opposed to ch.37 and ch.39 where Jeremiah accuses prophets of falsehood, here Jeremiah seems unsure whether to believe Hananya. Why? What is it about Hananya that seems authentic? How can one identify a false prophet? And why does Jeremiah need an explicit directive by God to identify Hananya's prophecy as false?