Revealing mistake: As Mr. Gold bangs his head on the table after being spritzed with pepper spray, you can see that one performing this action is Robert Carlyle's stunt double, and that the stuntman merely hits the table with his hands. (00:10:20)
Revealing mistake: When David is letting Henry drive his truck, there is a close-up of Henry, followed by a wide-shot of the car where the tires are screeching, and you can see that the one who is actually driving is Jared Gilmore's stunt double. (00:30:15)
Revealing mistake: As Killian Jones and Rumplestiltskin duel, there is an overhead shot of them fighting, and in the next shot, you can see that Robert Carlyle and Colin O'Donoghue have been replaced by their stunt doubles. (00:17:35)
What Happened to Frederick - S1-E13
Revealing mistake: At Granny's, when Emma and August ride off on the motorcycle, you can see that the one driving is actually Eion Bailey's stunt double. (00:21:45)
We Are Both - S2-E2
Revealing mistake: When Regina is trying to escape on horseback, as she says "Come on, Rocinante, go!", you can see that the one who's actually riding the horse is Lana Parrilla's stunt double. The stunt double's face can also be glimpsed when Regina is caught by the tree branches, and when she drops to the ground. (00:05:20)
Snow Falls - S1-E3
Revealing mistake: When Snow White steals one of the royal guards' horses, as she rides off, in the second shot, you can see that the one riding is not Ginnifer Goodwin, but her stunt double. When Prince Charming catches up to her and knocks her off her horse, Josh Dallas' and Ginnifer Goodwin's stunt doubles can both be seen. (00:03:55)
There's No Place Like Home - S3-E22
Revealing mistake: After Snow White hugs Red Riding Hood goodbye, there is a wide shot where Snow walks toward Charming. If you look at her face, it it not Ginnifer Goodwin, but a body double. The body double's face can also be seen in the wide shot where Snow walks away. (00:19:20)
Revealing mistake: When Rumplestiltskin is screaming at Snow and Charming about their unborn child's name, he speeds up for a few seconds and looks comically fast forwarded.
Snow Drifts - S3-E21
Revealing mistake: When future Hook is trying to stay hidden while Emma is making out with past Hook, there is a wide shot where future Hook sneaks past a corner, and you can see the face of Colin O'Donoghue's body double. (00:33:20)
Revealing mistake: At the end of the episode, when Emma goes to Gold to ask for his help, we can see he's wearing a wedding ring on his left hand in addition to the usual ring with the light blue stone he wears on the right. That's actually Robert Carlyle's own wedding ring; as he wrote during a twitter Q&A session, he forgot to take it off before they started to film. (00:40:35)
We Are Both - S2-E2
Revealing mistake: When Mary Margaret tries to escape and Mulan uses a sling to bring her down, you can see that the one who falls to the ground is Ginnifer Goodwin's stunt double. (00:39:50)
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.