Continuity mistake: At Lake Nostos, in the first shot of Emma, Snow and Mulan, there is a giant rock on their left. None of them move, but two shots later, the rock is on their right.
Continuity mistake: At Lake Nostos, when Emma says "You're not going anywhere. This portal's taking us home", in the wideshot, Snow's quiver strap is over her pink cardigan. In the close-up, it is under the cardigan. In addition, Mulan's and Snow's weapons are held at different angles between shots.
Tallahassee - S2-E6
Continuity mistake: When Emma cuts the trip wire in Anton's castle, the surroundings change completely between shots; for example, as she cuts the wire, there is an open chest near her. In the overhead shots, the chest is gone, and Jack's skeleton is missing. (00:33:50)
Continuity mistake: When Snow is dumped by the road by King George's men, the amount of dirt on her clothes keeps changing between shots. (00:14:15)
Continuity mistake: When Mary Margaret is talking to Regina in the school yard, her fringe keeps changing between shots. (00:08:00)
The Return - S1-E19
Continuity mistake: When Emma confronts Sidney, the listening device that she places on the table keeps changing position between shots. (00:15:15)
Continuity mistake: When Emma and Jefferson are struggling over the gun, the gun keeps changing position on the floor, without anyone touching it. (00:34:00)
What Happened to Frederick - S1-E13
Continuity mistake: At Lake Nostos, Charming drops his cloak and scarf by the shore. In the following shots, the garments keep changing position on the ground. (00:20:00)
True North - S1-E9
Continuity mistake: When Hansel and Gretel are outside the gingerbread house, Hansel dips his finger in some icing on the wall, which leaves a mark. Two shots later, more of the icing is missing, even though no one touched it. When Gretel climbs onto the window sill, the icing is fully coated. (00:17:50)
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)
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 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 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)
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)
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)
Snow Falls - S1-E3
Continuity mistake: As the trolls search through Prince Charming's belongings, one of them is seen pulling out Snow White's wanted poster from the bag, but in the previous overhead shots, he was already holding it in his hands. (00:27:45)
The Thing You Love Most - S1-E2
Continuity mistake: When Emma is walking Henry to school, there are several instances where they walk past the same building more than once. For instance, as Henry says "Luckily, I have a plan", they pass a shop with a wooden rudder on display in the window. When he says "Don't let her see these pages", they walk past the same shop again. (00:12:40 - 00:14:00)
The Thing You Love Most - S1-E2
Continuity mistake: When Regina is talking to Sidney by the apple tree, as Sidney says "Bounced around all over", Regina is picking an apple. In the next shot, she's already holding it in her hands. (00:11:10)
Tallahassee - S2-E6
Continuity mistake: As Snow and Mulan fight, Snow's pink cardigan gets covered in straws. The amount and position of the straws keep changing during the fight, and afterwards. Also, when Snow tackles Mulan, in the first close-up of them fighting, you can see straws from previous takes on Snow's cardigan, even though she has not come into contact with the ground yet, and the cardigan was perfectly clean in the previous shots. (00:37:40)
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.