I'd say he started having suspicions about it after the lemon madeleine. Right now I don't remember exactly what other occasions convinced him of Neuta being his neighbor, but I'm sure it started from there
He heard the sound of his mom's car Hoon from Neuta's side while they were having a call if I remember correctly? The Madeline made him have suspicions but while Neuta was redoing his character, they were in a call and he heard the car hoon.
He heard the sound of his mom's car Hoon from Neuta's side while they were having a call if I remember correctly? The Madeline made him have suspicions but while Neuta was redoing his character, they were in a ... urmom
This, and he further confirmed this when Lee Yeo Woon was in his house and chatted with him using his game account.
How did ji9star know that lee was neuta??? IM GENUINELY CONFUSED, LIKE WHEN DID IT HAPPEN AND STUFF-