Continuity mistake: When Emma wakes up on Jefferson's couch, her left tank top strap changes position between shots: First, it's at the edge of her shoulder. When she sits up, it's off her shoulder. In the next shot, it's on top of her shoulder, before moving back to the edge of her shoulder in the following shot. A couple of shots later, it's at the top of her shoulder, even though her hands are tied behind her back, and she could not have moved it. (00:10:20)
Continuity mistake: When the Mad Hatter and The Evil Queen are in the portal of doors, on the left side of the mirror, there's a door with green curtains. The design of the curtains changes between wide shots and the shot where they enter Wonderland. Most notably, in the wide shots, the curtains have brown edges, but when they are standing in front of the mirror, the edges are green. (00:15:55)
Continuity mistake: When Jefferson hands Emma the scissors, the sharp end is pointing towards Emma. However in the next shot the sharp end is pointing in the opposite direction. (00:24:35)
Continuity mistake: In the Queen of Heart's maze, when Regina closes the hedge to entrap the pursuers, the spears sticking out from the leaves change position between the wide shot and the close-up. (00:26:10)
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)
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.