Continuity mistake: When Mr. Gold and Belle are walking to the pawnshop together, their position in relation to the storefronts they walk past keeps changing. Most notably, they walk past a couple of wheelbarrows three times in consecutive shots. (00:13:35)
Continuity mistake: Aboard The Jolly Roger, Baelfire's shawl, on the bench, changes position between the close-up and the wide-shot when Hook puts it down. The shawl changes position again between when he says "Now, what makes you think you are any different?", and when Belle grabs it. (00:26:10)
Continuity mistake: When Archie's friends are gathered in Mary Margaret's apartment after the funeral, the cutlery in the bowls of salad moves around between shots when Ruby is talking. (00:12:50)
Continuity mistake: When Belle is looking for Baelfire's shawl and distractedly sets down the gun, the gun keeps changing position on the bench. (00:24:00)
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.