•Well he had clues since the starting for many chapters. But the final strong clue was in CHAPTER 37 where ml texts the game character but mc gets the message while he his next to him in real life.
•Then in CHAPTER 44 mc (neutaa) tells ji9star, the ml, that he is sick and then ml comes to check up on real life mc.
•We got male lead's side of the story in CHAPTER 54 and final confirmation that he ACTUALLY knows. But even before that he showed signs that he already knew so you would've already figured it out and it's not really one single chapter but this was the final sure confirmation chapter for those who had doubt.

I left off when the ml (idr names Dx) figured out who the mc was
How far is it since then? :o