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)
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.