It's not that hard to figure out especially if you are aware of Woohyung personality. He is cold to everyone, so even a slight showing of affection is huge, and Jaemin slowly realized this as Woohyung only blushes in front of him, he comes to his room frequently to see him, he excessively worries about him when he is hurt, the only human photo he ever took is Jaemin - all this stuff pile together and it's not hard to figure out, especially Woohyung hardly ever gets close to anyone. I actually find it refreshing that we have a main character that picks up all the clues instead of the usual hyper dense protagonist.

I really love this!! I love the fan-whore mc he’s such a cutie. And maybe also a PR god?
…but I gotta be honest, I’m not feeling the love here. There’s like zero romance. And I don’t even know when or how Jaemin figured out Woohyung had a crush on him?? I swear one day he just decided it was himself, so ever since he just tries to discourage him