Visible crew/equipment: When Regina and Hood see each other for the first time in the morning and Hood "apologizes" to Regina you see in the mirror in the background a blonde woman watching the scene. (00:07:40)
Continuity mistake: When Will is trying to break into the library, the sign on the door, with the opening hours, is blue. When Will and Robin enter the library, as the door closes, the sign is black. (00:35:15)
Answer: He knows who she is because he knows that the fairy tales are true. He was also shown to sneak out to go see Emma or someone else. Sometimes he succeeded, sometimes he didn't. This is how he knew to look for her despite his strict mother.