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: The way the baby blanket is wrapped around the infant Emma repeatedly changes between shots. For example, as Charming approaches the magical wardrobe, the blanket has just a tiny opening over the baby's face, and the embroidery with Emma's name on it is showing; in the next shot, the opening is much bigger, and the embroidery can no longer be seen. (00:30:00)
Continuity mistake: When Henry is in Emma's apartment, the amount of juice in the bottle repeatedly goes up and down between shots. (00:08:40)
True North - S1-E9
Continuity mistake: When Emma and Mary Margaret are conversing on the bed, Emma's sweater moves up/down her left shoulder several times. For example, as Emma says "Apparently that book you gave him, aren't exactly the stories in the most traditional sense", the sweater is over her shoulder. In the next shot, it's off her shoulder. In the following shot, it's covering her shoulder again. (00:37:45)
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)
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.