Continuity mistake: After Henry runs up the stairs, he puts his backpack on the bed, with the backside facing upwards. When he takes off his shoe and throws it on the floor, the top of the backpack is facing upwards. When Regina enters his bedroom, the backside is facing up again. (00:38:35)
Continuity mistake: When Henry lies on the bed, the position of his legs goes from one leg being crossed over the other, to being parallel to each other, between shots. (00:38:40)
Continuity mistake: When Sean is visiting Ashley at the hospital, as he says "Yeah, I'm back", baby Alexandra is waving her tiny hands in the air. In the previous and following shots, her hands are down. (00:39:10)
That Still Small Voice - S1-E5
Continuity mistake: During Henry's first therapy session, as Archie says "So all the crickets in Storybrooke, they were once people too?", Archie's holding a coffee cup in his hand. In the next shot, the cup is sitting on the table. (00:02:45)
That Still Small Voice - S1-E5
Continuity mistake: After Regina tells Emma to clear the crowd away after Henry is rescued from the collapsed mine, Regina's hand goes from being on Henry's face to his side/waist between shots. (00:34:35)
The Shepherd - S1-E6
Continuity mistake: When the twin brother is getting ready to slay the dragon, you can see the scar on his face which hasn't happened yet. In scenes before and after, the scar is not visible. (00:24:00)
The Shepherd - S1-E6
Continuity mistake: When Mary Margaret is sitting at the bar with Doctor Whale, the straw and lemon slice in the empty drink in front of her keep changing position between shots. (00:39:40)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: When Graham is arguing with Emma at the beginning of the episode, in the very first shot outside Granny's, they are approaching a shop called Worthington's Haberdashery. As the shot ends, they are still a few meters away from it, but in the next shot, they are already in front of it. (00:01:50)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: When Emma is talking to Mary Margaret in the morning, as Emma says "Tell me, what do I do?", Mary Margaret's left hand jumps from the bowl of sugar to her teacup between shots. (00:09:40)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: In beginning of the scene where Regina pays a visit to the Sheriff Department, the darts on and around the dartboard change position between the close-up of the dartboard, and the wide shot where Emma bends down to pick up a dart. (00:22:05)
The Heart is a Lonely Hunter - S1-E7
Revealing mistake: When Emma and Graham are chasing after the wolf into the graveyard, there is a shot where the wolf turns its head toward them, closes its mouth and perks up its ears, while growling. The wolf howls, before turning its head toward them again, and you can see that it is exactly the same shot that was used earlier, only this time, the wolf can be heard whimpering. (00:28:40)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: As Emma exits the crypt, in the wide shot, Regina's left hand, holding the flowers for her father's grave, is at her waist. In the next shot, her hand is down by her side, before she brings it up to show the flowers. (00:33:35)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: After Emma punches Regina, Regina has a wound by her mouth. For the rest of the episode, the wound keeps changing appearance and position. (00:34:40)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: When Emma is with Graham at the Sheriff's office at the end of the episode, her hair keeps changing. For example, in some shots, there is a lock of hair hanging down the left side of her head. In others, it is tucked behind her ear. (00:36:15)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: When Emma is pressing an ice pack to the wound that Regina gave her, her bracelets repeatedly move up/down her arm between shots. (00:36:25)
Revealing mistake: After the scene with Emma and Henry at the playground, there is a wide shot of Storybrooke Main Street. As the camera pans down, in the upper right hand corner, you can see the clock tower, but without the top two stories (the top two layers of the clock tower, including the clock, are CGI). (00:06:15)
Continuity mistake: When Mr. Gold brings the town charter to Mary Margaret's apartment, he puts the binder on the table twice; once before he sits down, and then again after he sits down. (00:19:10)
Continuity mistake: When Emma goes to get the fire extinguisher, Regina grabs Emma's arm at the elbow. In the next shot, she's gripping Emma's wrist. In the following shot, she's gripping her elbow again. (00:22:25)
Continuity mistake: When Regina is on the stretcher, she has a yellow coat around her shoulders. As she pushes Sidney away, the coat has vanished into thin air. (00:23:50)
Continuity mistake: In the scene where Morraine is conscripted into the army, as well as the scene where the knights find Rumplestiltskin and his son on the King’s road, Rumplestiltskin's hand repeatedly moves up/down his staff between shots. The same thing happens to Baelfire in the scene where he and his father are preparing to leave for the Duke's palace.
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.