We didn't get it in the movie, but I'd love for Queenie to be the one to figure out Graves' real identity. Perhaps she sees through him right away, or perhaps it starts out as "something isn't quite right..." and progresses. Bonus points off this culminates in a mind duel of some kind. Extra bonus if he's impressed by her power and tries to get her to join him.
Queenie vs. Graves, legilimancy