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)
Continuity mistake: In Neverland, the piece of cake that Rumplestiltskin wishes for changes during this scene, with regard to the amount of chocolate, white cream, etc, and also its position on the wood after Rumple puts it down. (00:16:45)
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.