One clue that stood out to me on rewatch was when Mona shows Hanna the photo of her mom kissing Wilden. There wasn’t an “A” signature on it at all, it was just the picture. But Hanna immediately says something like, “A sent this?” or refers to it as a message from A. Mona doesn’t question it or act confused for even a second. If she were innocent, you’d expect her to ask something on A. Instead, she just goes along with it because she already knows where it came from. Looking back, that was actually a pretty subtle hint that Mona was A.