The Heart of the Truest Believer - S3-E1
Continuity mistake: After Neal wakes up on the bier, after he says "A .45 caliber arrow", Aurora's dress keeps alternating between having straps and being strapless between shots. (00:13:25)
Continuity mistake: When Emma and Hook kiss, in the shots from behind Hook, Emma tilts her head to the right. In the shot from behind Emma, she tilts her head to the left. In addition, Emma's left hand jumps from the back of his head to his shoulder between shots. (00:37:10)
Continuity mistake: In Neverland, when Rumplestiltskin's father tells him to think of a food, a slice of cake appears in the palm of the boy's hand. When Rumple walks to place the cake down, suddenly there's a round, white doily under the slice, but when he sets the cake on the wood the doily is gone again. (00:16:50)
There's No Place Like Home - S3-E22
Continuity mistake: When Prince Charming shows Snow White the wanted poster, the poster does not have Snow White's name on it. Two shots later, he's still holding it up, but now the poster has her name printed on it. (00:05:30)
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.