This question deals with the ending of Fantastic Beasts and Where to Find Them. So the most important stuff will be behind spoiler tags, but it's really one big spoiler.
At the end of the story, Newt manages to disarm Graves and reveals that
Graves is actually Gellert Grindelwald in disguise and apparently has been this entire time.
I saw no indication throughout the story that Graves was anything other than what he claimed to be. This whole scene seemed to come out of nowhere.
Why would Newt have even suspected it? Were there any clues?