The Thing You Love Most - S1-E2
Factual error: This episode takes place the day after Emma decides to stay in Storybrooke and time started moving. According to "The Stranger" and "Selfless, Brave and True", August woke up in pain in Phuket on October 24, 2011 at 8:15am, the exact minute that Emma decided to stay in Storybrooke at 8:15pm. Since Thailand is 12 hours ahead of Maine's timezone, this means that time started moving in Storybrooke on October 23, and that this episode takes place on October 24. During Regina's conversation with Emma in her office, she says, "Did I know that my son comes to my office every Thursday at precisely 5pm so I can take him for dinner before his therapy session? Of course I did." However, October 24, 2011 was a Monday. (00:30:05)
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.