How did Annie know Armin was lying? When Armin approaches Annie in an alley to ask her to help Eren escape, Annie agrees but puts on her ring (with the blade). Armin later realizes that she knew from that moment that he was lying. How?
The answer is in chapter 31.



Annie noticed that Armin is not looking at her like before. Maybe she detected hints of fear on Armin's eyes, which means Armin knows that Annie is the Female Titan.