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)
The Return - S1-E19
Continuity mistake: When August drives off on his motorcycle, in the close-up of his foot as he walks, the pavement he steps on has concrete paver blocks. In the next shot, where he sits on the motorcycle, the motorcycle is parked in front of the Storybrooke Pet Shelter, and the pavement has changed to asphalt concrete. In the next shot, the vehicle, seen from a different angle, is parked in front of the business located on the left side of the pet shelter; easily seen from the pavement, which is now paver blocks again. (00:16:45)
The Return - S1-E19
Continuity mistake: When Mr. Gold is talking to Dr. Hopper in his office, there's a box of tissues on the table. The amount of tissue paper sticking up from the box keeps changing between camera angles. (00:26:15)
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.