Revealing mistake: In real life, the Storybrooke Cannery is known as the Gulf of Georgia Cannery, a real cannery in Steveston village, the town which doubles as the town of Storybrooke on Once Upon a Time. In most episodes where the cannery appears, the name on the building is replaced with the Once Upon a Time version. However, in this episode, when Emma is driving in her father's truck following shortly behind Cruella's car, there is a wide shot of Cruella's car turning right, and you can see the sign on the cannery, which has not been updated properly: Bizarrely, the first word of the real business name is still on the building, and the first three letters of the fictional name are missing, making the name on the building "Gulfrybrooke Cannery." (00:23:20)
Revealing mistake: When Regina is waiting outside the Storybrooke Library, there is a wide shot of Cruella's car making its way down the road. On the building on the right side, you can see a large vertical sign on the side of the building, which lists several local businesses. If you read the top of the sign, you can see that it says "Steveston Hotel." Steveston is the Canadian town which doubles as the town of Storybrooke on Once Upon a Time. (00:20:45)
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.