Continuity mistake: When David is making breakfast, he turns around, with a frying pan full of pancakes in his hands. In the next shot, he begins to lift up a pancake with a spatula, but when it cuts, he's already holding it out. (00:03:15)
Continuity mistake: After Mary Margaret opens the birthday present from Johanna, the wrapping, lying around the present, changes position a couple of times. (00:03:55)
Revealing mistake: When Hook attacks Mr. Gold, if you watch as he pushes Emma out of the way, you can easily see that one who actually pushes Jennifer Morrison is Colin O'Donoghue's stunt double. When Hook pushes Mr. Gold against the security gate, it is the stunt double who pushes Robert Carlyle. (00:18:25)
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.