Continuity mistake: When Mary Margaret and Emma are walking down the street, as Mary Margaret says, "This is the best I can do", she begins to walk past the corner of a building. In the next shot, they begin to walk past it again. (00:05:15)
Continuity mistake: When Astrid accidentally dumps glitter on Leroy, in the first shot, there's hardly any glitter on him. Cuts to the next shot, and he is covered in glitter. The amount of glitter on his clothes repeatedly changes for the rest of the scene. (00:05:20)
Continuity mistake: In the scene where Emma is taking pictures of Kathryn's car and Sidney shows up, they open the trunk lid. None of them close it, and the camera is on them the whole time, but when David shows up, the lid is closed. (00:08:00)
Continuity mistake: When Bossy is cutting Dreamy's beard, Dreamy asks "Who was the woman I saw?" and Bossy stops cutting, but a couple of shots later, Dreamy's beard is suddenly much shorter. (00:10:00)
Continuity mistake: After the door to door candle sales fail, when Leroy is about to deliver the bad news to Astrid, the position of Mary Margaret's coat, which she's holding in her arms, changes between shots. (00:21:00)
Continuity mistake: In "The Shepherd", Kathryn is showing David their house, and you can see that the street number on the Nolans' house is 244. In this episode, when Emma is looking at Kathryn's phone records in the evening, the phone records list her address as number 4. (00:38:25)
Continuity mistake: Leroy is walking by and Astrid she accidentally dumps glitter on his head. She wipes his face off; he has glitter on his moustache. It disappears. The camera pans back to him and there is glitter on his moustache again.
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.