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)
We Are Both - S2-E2
Continuity mistake: After Mary Margaret is knocked out by Mulan and falls to the ground, the amount of straws and dirt on her clothes changes between shots. When she is thrown into the dungeon with Emma, her clothes are clean. (00:39:55)
We Are Both - S2-E2
Continuity mistake: When Regina's father, Henry, tells her about Rumplestiltskin, he refers to him as "someone Cora knew before I met her." However, "The Miller's Daughter" establishes that Cora first met Rumplestiltskin while she was locked up in King Xavier's palace, after meeting Prince Henry when she delivered the flour and at the masquerade ball. (00:10:45)
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.