Continuity mistake: At the beginning of the episode, when Emma and Mary Margaret are walking through the camp with a group of refugees, in the first shot, they're at the edge of the camp. Cuts to a different shot, and the group have moved to a different part of the camp and are situated next to a table, before they walk over to a shed. In the next shot, they are farther away from the shed. When it cuts, they are standing next to it again. (00:05:00)
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)
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.