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)
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.