Nah, it’s clear he started to suspect it by chapter 74 because of how she reacted to him while under her male disguise. You’ll notice he keeps eying her and touching her face like he’s waiting for her to betray herself so she can confirm his suspicions. Heck, before chapter 100, when they sleep in bed a few chapters beforehand, the way he looks at her makes it clear. Then he sees the scar left by the wound she got that first night they “met”!l and that seals it for him.
I mean sure, from chapter 74 and on, it’s only suspicions. With each chapter after that, those suspicions grow into the belief that they’re one and the same; all he’s missing is proof.
Maybe you understand Korean so that’s why you can say he knows for sure by chapter 95, but from what I can see, he believes his suspicions are validated in chapter 98 when he sees the scar she got that first night when he met her female self. Chapter 99, he talks to her/argue on it with her but she seems to be the one who pushes him away. Then comes chapter 100 when she gets jealous and he goes after her. She then transforms into her female self before his eye, confirming what he already knew by then (since he actually kissed her while she was under her male disguise lol).
If you go back and reread chapter 95 you'll see at the very end of the chapter she is riding with him on the horse and wakes up suddenly, she had passed out while riding I'm assuming due to her chronic injury on her arm. Then in maybe 9 there is a flashback scene while he is watching her sleep in the bed that he is remembering where she is riding the horse with him and she passed out and accidentally shifts back into her female self. That's when he knows for sure. They show the flashback twice, the second time is in chapter 100.
Yeah, its Chapter 96 at the end where he is having the flashback https://tkor.cloud/%EC%97%B0%EB%A1%9D%ED%9D%94_96%ED%99%94.html

When will he find out who she really is.
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Chapter 100
It is not clear when he really found out but in that chapter he kisses her while she is disguised as a man. Than she turns into a woman in front of him. He doesn't appear to be surprised so I think he know, but since when I have no clue(⌒▽⌒)