Julia visits Quentin and inadvertently reveals the illusion's falsity by mentioning she didn't see the "fireworks" Quentin cast—something she shouldn't have known about if he were truly non-magical.
Quentin leads a music therapy group in a rendition of Taylor Swift’s "Shake It Off" to annoy the "orderly" version of Penny, testing if the real Penny’s consciousness is linked to the dream. [S1E4] The World in the Walls
The Magicians: Season 1, Episode 4 - "The World in the Walls" [S1E4] The World in the Walls