Not yet
he knew that Hakyung was impersonating a dead woman, he even states that when they're in the "fighting" stage at the start. But his finding out of him being a man it's not clear. At this point, (in the flashbacks) he doesn't know or show any suspicion.
Spoiler
Ye later on he finds out that hakyung is actually a woman,its not explicitly addressed or told but there's a scene where yoosung becomes suddenly furious and his head is in his hands.He has found out he's been loving a man
do you remember which chapter that was?
Chapter 31, and I think ch 36/35 when he asks "is this another one of your roles?" Its heavily implied
yes.
Spoiler...
In the hotel, he noticed he was a man, which was why he started behaving like that.
I kinda forget so did we ever find out that Yoonsung knew Hakyung dressed as a women and impersonating a dead woman?