Continuity mistake: In the scene where the Sheriff of Nottingham (his Storybrooke counterpart) tries to apologize to Mr. Gold, they're at the corner of the Rabbit Hole, and in some shots, the clock tower building is seen behind the Sheriff. In other shots, there's a completely different building behind him. In the same scene, Mr. Gold alternates between standing right by the corner, and standing a couple of meters away. (00:35:40)
Continuity mistake: When Mr. Gold and Lacey are on a date at Granny's Diner, when Granny comes with their drinks, she moves her hand towards Gold's glass of ice tea, sitting on the tray. In the next shot, we hear the clinking of ice as Granny picks up the glass, as well as the sound of the glass being put on the table, off-screen, on Gold's left. In the next shot, the glass has mysteriously moved to Gold's right, even though the camera was on him the whole time, and he never moved it. (00:25:00)
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.