You can know which song it is by listening to the intro, which involves two factors: understanding and integrity.

First of all, being able to identify a song by listening to the intro requires us to have a certain understanding of the tune, melody, lyrics, etc. of the song. Only when we have enough knowledge of the song can we hear the details and characteristics of the song from a prelude, and thus determine its identity. This understanding is based on our overall understanding of the song.

Secondly, you can know which song it is by listening to the intro, which also involves the issue of integrity. An intro may only contain part of the song's information, but through our holistic cognition, we can match it with known songs to identify the song's identity.

Therefore, the ability to know which song it is by listening to the intro requires both our understanding of the song and our overall understanding of the song. Only when we have a complete understanding of each part of the song can we fully identify and judge it from a piece of incomplete information.