Someone please enlighten me

from what i remember he saw a notification on mc's phone, probably for the app that they text on lolol

Probably because mc talks a lot and he even used ml looks in the game its pretty fucking obvious in comparison ml doesn't talk too much

MAYBE SPOILER(ish)?
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A lot of things played a part in him finding out. Neuta brought up the madeleine he had received from his neighbour. He also said that he modelled his character's appearance after his neighbour. Afterwards, Ji9star heard the same sounds both outside his flat and during their call. He also knew that Neuta was sick and surprise, his neighbor was sick at the same time. He connected the dots and found out.

Iirc someone said it was during their voice call, jigu heard the announcement about his mother's car so he kinda figured it out. I guess it's from the novel rather than the manhwa itself

When Jigu and Yeowoon were out in the rain Jigu's mom made him give Yeowoon Madeleines. Yeowoon then later sent the picture to Jigu (jistar) online and thats when Jigu started suspecting Yeowoon to be Neuta

When Yeowoon was at Jigu's place he got a message the same time that Jigu sent a message to Neuta. Jigu asked him if the friend who messaged him was someone he met through a game and Yeowoon confirmed it.

I think that was the time Jigu also confimed that Yeowoon = Neuta

It's from the small things how he found out, like 1 the cookies and 2 his character model 3 him being sick 4 he he heard the same sounds in the vc, he just connected the dots and there we have it. Mc mentioned about playing games too so there's that info too.

After he gave him madeleines, he connected the dots. He knows since then.

Yeah and to not forget the fact that jigu had a stalket before so psychologically speaking he could be just extremely observant and wary