I have no idea how netizens know. Apparently they starred in Smile, Donghae together.
Article: Ji Chang Wook confesses to being in a secret relationship... the girlfriend is girl group member A?
Source: New Daily via Nate
1. [+134, -2] Ji Chang Wook usually takes 'leading' roles (TN: Jooyeon also means leading in Korean)
2. [+95, -6] a s j y
3. [+71, -2] Jooyeon