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)
There's No Place Like Home - S3-E22
Revealing mistake: After Snow White hugs Red Riding Hood goodbye, there is a wide shot where Snow walks toward Charming. If you look at her face, it it not Ginnifer Goodwin, but a body double. The body double's face can also be seen in the wide shot where Snow walks away. (00:19:20)
Snow Drifts - S3-E21
Revealing mistake: When future Hook is trying to stay hidden while Emma is making out with past Hook, there is a wide shot where future Hook sneaks past a corner, and you can see the face of Colin O'Donoghue's body double. (00:33:20)
Snow Drifts - S3-E21
Continuity mistake: When Emma changes clothes in the Enchanted Forest, as she puts on gloves, in the wide shot, she begins to put the glove on her left hand. In the following close-up, her left hand is bare, and she's putting the glove on her right hand. (00:18:15)
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)
It's Not Easy Being Green - S3-E16
Continuity mistake: When Hook is at the beach with Henry, after he says to him, "Fair enough, mate", the pendant on Hook's necklace goes from resting against his chest, to being partially tucked under his waistcoat. In the close-ups, the pendant is completely covered by the waistcoat. (00:26:50)
Continuity mistake: When Killian rallies his crew to join him in taking over the ship and becoming pirates, as he's holding onto the ropes used to control the vessel's sails, the number of ropes he's grasping repeatedly changes between shots; in some shots, it's only two ropes, in other shots, there are three ropes in his hand. (00:38:50)
Going Home - S3-E11
Continuity mistake: In the Neverland flashback, when Hook offers Tinker Bell the rum bottle, the stopper (attached to the bottle) is hanging over the back of his hand. In the next shot, it is not. (00:09:55)
Save Henry - S3-E9
Continuity mistake: When Regina talks to Rumplestiltskin, he is holding onto the bars in his cell. In the beginning of the scene, his left hand keeps clutching different bars between shots. After Regina says "Nothing can stop me now", his hands keep going from holding the bars and not holding them. (00:02:00)
Continuity mistake: In Neverland, the piece of cake that Rumplestiltskin wishes for changes during this scene, with regard to the amount of chocolate, white cream, etc, and also its position on the wood after Rumple puts it down. (00:16:45)
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.