
Now how the heck did he know that it was him?

He just knew. Karas said it himself, his laugh, words, mannerism, and the care that he felt from the fairy not knowing he is the 'Your Highness'. Then, to add the cherry on top, when he mumbled 'Apel', Abel instinctively responded to it. It wasn't just a sleeptalking thingy but rather his response to what he heard, unconciously. Habits die hard. Plus, the fact that Karas knew that the fairy doesn't know who he is yet but he called him 'Your Highness' while smiling fondly is a shoot shoot.
Wow this brings back memories LOL boys at my home town did that all the time like they were not able to express their immotions unless by bulling the girls they liked what a joke