Revealing mistake: When Killian Jones is dueling Rumplestiltskin and is pushed into some barrels, you can see that the one who actually falls down is not Colin O'Donoghue, but his stunt double. (00:18:20)
Revealing mistake: As Killian Jones and Rumplestiltskin duel, there is an overhead shot of them fighting, and in the next shot, you can see that Robert Carlyle and Colin O'Donoghue have been replaced by their stunt doubles. (00:17:35)
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.