Continuity mistake: When Elsa is walking down the middle of the street through Storybrooke, she is leaving a trail of ice on the road behind her. Shortly after seeing a wedding dress in a store window, she walks towards the store, and the trail of ice behind her is gone. (00:07:00 - 00:07:35)
Continuity mistake: During Emma's date with Hook, the waiter is carrying a basket of food. As Will bumps into him, the waiter is just about to put the basket on the table, but in the next shot, it is already sitting on the table.
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)
Continuity mistake: When Hook comes to see Emma at the sheriff's station, his earring disappears when she kisses him goodbye.
Continuity mistake: The flashback scene where The Snow Queen takes Emma's video camera away from Kevin differs from what is seen on the video recording of that moment in "Breaking Glass" and "Family Business." For example, in the recording, Ingrid says, "We respect property in this house, Emma." In the actual scene, she says, "We respect property in this household, don't we, Kevin?"
Continuity mistake: When Mary Margaret gets thrown by the Lily dragon into a rock, her blood smear on her head is large and messy. They cut away and cut back to her quickly and it is nice and neat with two blood drop lines slowly forming away from the cut. (00:44:00)
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.