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)
What Happened to Frederick - S1-E13
Revealing mistake: When Regina finds Henry with Emma at his playground In "Fruit of the Poisonous Tree", her license plate number is 40F5 TH. When the gym teacher finds Kathryn's car in this episode, his car has the same license plate, indicating that it is a reused prop. The same license plate appears on yet another car when August drives off with his motorcycle in "The Return", on Archie Hopper's car when he tries to cross the town line in "We Are Both", and on a parked car when Emma and the others arrive in Storybrooke in "Mother." (00:40:10)
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.