She said that he looked surprised when he began to read the history of the country like he didn’t know it
She also found also out that he "lost his memory" when she had someone do research into him for her and seemed to act very differently. There was also how he didn't seem to know what hobbies he liked, what subject he likes the most, he also said how he wouldn't know what to do anymore if he wasn't the Prince's guard anymore, he also didn't seem to react like everyone else once he learned about what the title she had really meant, and probably a few other small things as well.
Ever since she said she got the same vibes from him and the girl and then it was basically confirmed when they were studying and he had trouble answering questions about his past (also it seemed like maybe he couldn’t read?)