Mike, Selene, Kotone and Dawn find out Giselle is not the real Sylvia because the former two saw them swap places, while the latter two discovered her due to their powers.
Also, Danny finds out that Sylvia is not the real Giselle because Giselle never double checks, she doesn't act all lone wolf or cool to people she meets like Liz does, her earrings are not dangling silver bones, and she never waves with her right hand because she's left handed. Also, he didn't feel any connections to her.
