Trivia: In the scene at the airport where Mr. Gold, Emma and Henry are standing in line for the security check, the three children waiting right in front of them are Robert Carlyle's own children, acting as extras. (00:16:45)
Trivia: The tie Henry chooses to go with the suit he'll be wearing to go to work for his grandfather is identical to the one Gold himself wore in Season 1 Episode 8 "Desperate Souls." (00:15:25 - 00:41:10)
Trivia: Every episode intro you can see something hidden in the intro like a building or a creature or a person. This is the same as Once Upon a Time in Wonderland (2013).
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.