Continuity mistake: After Emma pulls Hook out of the tub of water, the wet hair plastered to his forehead keeps changing from shot to shot. (00:11:30)
Continuity mistake: After Emma gives Hook mouth-to mouth, her left hand keeps changing position: Most noticeably, as she whispers, "Hook, come back to me", her hand is on his chin. In the next shot, her hand is resting on his chest. (00:11:50)
Continuity mistake: When Zelena loses the Dark One's Dagger, it lands on the ground, the patterned side of the blade facing upwards. When Regina picks up the dagger, the patterned side is facing downwards. (00:28:10 - 00:29:20)
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.