Save Henry - S3-E9
Continuity mistake: When Regina talks to Rumplestiltskin, he is holding onto the bars in his cell. In the beginning of the scene, his left hand keeps clutching different bars between shots. After Regina says "Nothing can stop me now", his hands keep going from holding the bars and not holding them. (00:02:00)
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.