Welcome to Storybrooke - S2-E17
Continuity mistake: Baelfire/Neal is in the diner and wraps his scarf around his neck while Emma walks over to him. Then the camera is on Emma and you can see Baelfire/Neal moving as he puts the scarf on. Seconds later the camera switches back to Baelfire/Neal and he wraps the scarf around his neck a second time. (00:25:05)
Continuity mistake: In the scene where Rumplestiltskin comes home and discovers that Milah is not there, the wool he's carrying keeps changing position in the basket. (00:02:10)
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.