Visible crew/equipment: After Emma and Henry meet Archie in the streets of Storybrooke, as Emma opens and closes her car door, a reflection of a white lighting screen and a camera can be seen on the door. (00:17:00)
Snow Falls - S1-E3
Visible crew/equipment: When the horse carriage is ambushed, just as Princess Abigail can be heard screaming "Please, help me", the camera pans downward and a horse wrangler wearing a blue baseball cap can be seen in the lower left hand corner, hiding behind one of the horses. (00:03:20)
An Apple Red as Blood - S1-E21
Visible crew/equipment: After Regina walks into Gold's shop and tells him about the sleeping curse, just as she asks, "Don't you understand?" we can see the green tape mark (and its reflection) on the floor in front of the display case, where Regina is about to stand. (00:38:55)
The Miller's Daughter - S2-E16
Visible crew/equipment: When Rumpelstiltskin goes to Cora as she is trying on her wedding dress, as he sits down on the chair behind her, you can see the tape used to mark the spot on the floor - it's bright red and in an X shape. (00:24:39)
Visible crew/equipment: When Regina and Hood see each other for the first time in the morning and Hood "apologizes" to Regina you see in the mirror in the background a blonde woman watching the scene. (00:07:40)
Tougher Than the Rest - S6-E12
Visible crew/equipment: When Sir Henry searches for his mother and The Evil Queen along the water's edge, in the shot where he says, "They crossed the river", as he turns his horse around, a horse wrangler's hand can be seen holding the horse's bridle in the lower half of the screen, guiding the horse to turn around. (00:04:20)
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.