Cora: Love is weakness. It feels real now, at the start it always does, but it's an illusion. It fades and then you're left with nothing!
Mr. Gold: She now thinks she's a scantily clad barfly named Lacey. She has a false life, like you did under the curse with Kathryn, only hers apparently involves a lot more alcohol.
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.