Continuity mistake: After Emma pulls Hook out of the tub of water, the wet hair plastered to his forehead keeps changing from shot to shot. (00:11:30)
Continuity mistake: After Emma gives Hook mouth-to mouth, her left hand keeps changing position: Most noticeably, as she whispers, "Hook, come back to me", her hand is on his chin. In the next shot, her hand is resting on his chest. (00:11:50)
Continuity mistake: When Zelena loses the Dark One's Dagger, it lands on the ground, the patterned side of the blade facing upwards. When Regina picks up the dagger, the patterned side is facing downwards. (00:28:10 - 00:29:20)
Snow Drifts - S3-E21
Continuity mistake: While Henry and his family are gathered around the table at Granny's, the amount of juice in the glass sitting on the table keeps rising and falling. (00:06:50)
Snow Drifts - S3-E21
Continuity mistake: When Emma changes clothes in the Enchanted Forest, as she puts on gloves, in the wide shot, she begins to put the glove on her left hand. In the following close-up, her left hand is bare, and she's putting the glove on her right hand. (00:18:15)
Snow Drifts - S3-E21
Continuity mistake: At the docks, when past Hook lifts Emma up, as he says, "Oh, no need", Emma is on her feet, facing him as he begins to pick her up. In the next shot, Emma is facing a different direction, her back leaning against his shoulder/arm as he picks her up and starts carrying her. (00:32:10)
Snow Drifts - S3-E21
Audio problem: As past Hook carries Emma aboard his ship, he drunkenly and loudly declares, "Behold! The Rolly Joger." Although they are several meters away from the camera, you can see that his lips are barely moving. (00:32:15)
Snow Drifts - S3-E21
Continuity mistake: When Emma is making out with past Hook while future Hook skulks around the room attempting to stay hidden, in the second shot, Emma's back is facing the stairs/ladder, her hand gripping Hook's coat. In the next shot, her back is facing the table, and her hand is on Hook's shoulder. (00:33:20)
Snow Drifts - S3-E21
Revealing mistake: When future Hook is trying to stay hidden while Emma is making out with past Hook, there is a wide shot where future Hook sneaks past a corner, and you can see the face of Colin O'Donoghue's body double. (00:33:20)
Snow Drifts - S3-E21
Continuity mistake: Aboard The Jolly Roger, when Emma is keeping past Hook occupied while future Hook skulks around the room, in a wide shot, future Hook sneaks past a corner. In the next shot, he begins to sneak past the very same corner again.
There's No Place Like Home - S3-E22
Continuity mistake: When Prince Charming shows Snow White the wanted poster, the poster does not have Snow White's name on it. Two shots later, he's still holding it up, but now the poster has her name printed on it. (00:05:30)
There's No Place Like Home - S3-E22
Revealing mistake: After Snow White hugs Red Riding Hood goodbye, there is a wide shot where Snow walks toward Charming. If you look at her face, it it not Ginnifer Goodwin, but a body double. The body double's face can also be seen in the wide shot where Snow walks away. (00:19:20)
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)
Deliberate mistake: Elsa's dress in Season Four is different from the dress she was wearing in the Season Three finale. The new dress is a different shade of blue, and has a completely different design, even though the present day events of Season Four take place mere moments after the end of Season Three.
Revealing mistake: Ruth has gray, curly hair. When Bo Peep bangs The Shepherd's crook on the ground, sending Ruth and David flying to the ground, in the shot from behind them, Ruth's hair is much smoother; the one falling is obviously Gabrielle Rose's stunt double.
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.
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)
Continuity mistake: When Hook comes to see Emma at the sheriff's station, his earring disappears when she kisses him goodbye.
Revealing mistake: When Mary Margaret and Regina engage in a sword fight in the sheriff's office, in several wide shots, the faces of Ginnifer Goodwin and Lana Parrilla's stunt doubles are visible. For example, in the wide shot where Regina is bending Mary Margaret across the desk and David says, "Careful", Lana Parrilla's stunt double is seen. (00:30:20)
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.