Quiet Minds - S3-E15
Continuity mistake: When the Vault of the Dark One opens, there is a wide shot where Neal turns around, and you can see that the area around the circular vault is covered in snow. In the next shot, a large area around the vault is free from snow. The amount of snow around the vault keeps changing during the rest of the scene. (00:26:45)
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.