1.) Justin's song "Beauty and a Beat" : In this song he says that he needs a "Beauty and a Beat" to make his life complete. On the other hand, Selena Gomez performed a song named Beat. She compared herself to the beat that Justin needs.
2) In her song Nobody does it like you, she says " sexy kinda swag "; and we know swag is the word that is only used to describe Justin Bieber.
3) In his song "Beauty and a Beat" It is clearly said in Nicki Minaj's rap "Selener" or simply, Selena.