He realized way in the beginning like when WWX saves the disciples, he knows because of the flute song. WWX the only person who knows it, in the book it's chapter 9-10
in chapter 23, when wei wuxian played the flute he remembered the melody of the song that lan wangji hummed in the xuanwu cave. the title of the song is wangxian, well its their song hahahhaha
PS. that's why lan wangji replied "You said it yourself" when wei wuxian asked how did lan wangji recognize him. i think that scene is in chapter 52.
Can somone tell me which chapter lang wangji relize mo xianyu is wei wuxian?