Revealing mistake: Ruth has gray, curly hair. When Bo Peep bangs The Shepherd's crook on the ground, sending Ruth and David flying to the ground, in the shot from behind them, Ruth's hair is much smoother; the one falling is obviously Gabrielle Rose's stunt double.
Revealing mistake: When Mary Margaret and Regina engage in a sword fight in the sheriff's office, in several wide shots, the faces of Ginnifer Goodwin and Lana Parrilla's stunt doubles are visible. For example, in the wide shot where Regina is bending Mary Margaret across the desk and David says, "Careful", Lana Parrilla's stunt double is seen. (00:30:20)
Revealing mistake: When Emma opens the door of Any Given Sundae in the 2011 flashback, you can see the backdrop curtain used to insert the Storybrooke main street, thanks to its shadow in the middle of the road and a few folds reflecting the light.
Unforgiven - S4-E13
Revealing mistake: When Snow White and Prince Charming arrive home at the Royal Castle on horseback, you can easily see that the ones riding are not Ginnifer Goodwin and Josh Dallas, but their stunt doubles. (00:02:55)
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)
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: After Mr. Gold's group leaves Mary Margaret's apartment, in the wide-shot where Mr. Gold asks Cruella and Maleficent to follow Regina, a direction sign pointing to a shop called "Steveston General Store" can be seen on a building. Steveston is the Canadian town which doubles as the town of Storybrooke on Once Upon a Time. (00:14:30)
Revealing mistake: When Snow White and Prince Charming hurriedly make their way through the woods in the beginning of the episode, in the shot where Snow White jumps over a log, you can see the face of Ginnifer Goodwin's and Josh Dallas' stunt doubles. Josh Dallas' stunt double can also be seen as Snow and Charming are running thought the woods in the previous shot.
Revealing mistake: During the car chase scene, when Lily is driving Emma's car, there are several wide shots of the car driving down the road and you can see that the one driving is not Agnes Bruckner, but her stunt double. Lana Parilla's stunt double can also be seen in several wide shots when Emma and Regina catch up with Lily and try to overtake her. (00:30:55)
Revealing mistake: In the first shot of the scene where Emma and the others arrive in Storybrooke in their cars, on the left side of the screen, you can see a sign behind a branch. It says "Steveston Museum and Visitor Centre." Steveston is the name of Canadian town which doubles as the town of Storybrooke on Once Upon a Time. (00:07:00)
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.