In season 1 episode 20, Armin figured out that Eren might be in the rear of the centre rank after the fight with the female titan. After that, he and Mikasa never met up.
However, about 15 minutes into the episode, Mikasa was thinking about something in the forest and recalled that Armin mentioned that Eren would be in the rear centre rank. But that makes no sense given they never met up after Armin figured it out and so Armin could never have told her.
So how could Mikasa know that Armin figured out that Eren was in the rear centre rank?