She knew pretty much from the very beginning.
She suspected him of being a human since her first appearance at the end of chapter 9.
In the next chapter we see her notebook with things that tipped her off. Unlike most other students, she knew that humans weren't just a fairy tail, so that's why she ended with a theory that Iruma is a human.
At the end of the same chapter she had a "fateful encounter" with Iruma (the kind she saw pictured in human manga), that pretty much cemented her belief that he was a human (and she though she was flustered, because he was using some "human magic" on her - flawed logic, but she was spot on).
The next chapter she pretty much confirmed it, when Iruma reacted to the manga book that Ameri dropped. After that, she had him read the manga, instead of reporting it to her father, as she initially planned.
PS: At this point of story, we know that these characters know (or highly suspect that) Iruma is a human - Sullivan, Opera, Alikred, Balam, Ameri, Bachiko, Kirio, Henri, Kalego, Narnia, Ocho, Baal.
Nope, no misunderstanding, she knows he's a human for a fact.
Nah, it was suspicion for less than a chapter, and a firm belief for less then another. After she learned he could read human manga, she didn't even need him to confirm it.
