Continuity mistake: At Lake Nostos, when Emma is pinned down by Hook, the ground beneath her changes between shots, with regards to the markings in the sand (from the fight), as well as the amount of stones, twigs etc. (00:29:00)
Continuity mistake: When Mary Margaret and Emma attack Cora, she vanishes into thin air. In the next shot, they run toward the portal, and their weapons have mysteriously disappeared. They were there in the previous shot, and there was no time to get rid of them, especially not Mary Margaret's quiver, which was strapped to her body. (00:30:50)
Continuity mistake: During the showdown at Lake Nostos, Mulan's long ponytail alternates between hanging down her back to being in front of her shoulder between shots. Emma's hair also keeps changing position. (00:27:30)
Continuity mistake: At Lake Nostos, the pendant on one of Emma's necklaces alternates between being hidden behind her neck (under her jacket) and being visible on her chest. This is seen, for example, when Cora fails at ripping her heart out. (00:27:45)
Continuity mistake: During the battle at Lake Nostos, after Emma lunges at Hook and he flips her to the ground, the amount of dirt on her clothes repeatedly changes. (00:28:00)
Continuity mistake: After Regina places a rose on Cora's body, the position of the green leaves repeatedly changes between shots. (00:22:05)
Continuity mistake: When Emma is trapped inside the cell and is gripping the bars while talking to Hook, she's holding different bars between shots. This happens several times. (00:14:50)
Continuity mistake: Right before Emma and the other women get trapped inside the cell, Emma is reading the scroll left by Rumplestiltskin. The way she holds the paper keeps changing between shots. (00:13:40)
Continuity mistake: When Hook attempts to rip Cora's heart out, as he pulls his hook out of her chest, the hook is pointing downwards. In the next shot, it is pointed to the right. (00:12:55)
Continuity mistake: When Hook kills Regina's guards, the first guard can be seen collapsing. Cuts to an overhead shot, and he collapses again. (00:00:50)
Continuity mistake: During Emma's sword fight with Hook, when Emma is on the ground, reaching for her sword, Hook sticks his own sword into the sand so that he can drag her away from her weapon. Cuts to a different shot, and his sword is lying on the ground, near his feet. A couple of shots later, the sword is planted in the ground again. (00:27:55)
Continuity mistake: After Hook throws the satchel that holds Aurora's heart to Mulan, the way she holds the satchel keeps changing; most noticeably, in some shots, she's holding it by the straps, in others, the satchel is lying in her hand. (00:28:20)
Continuity mistake: When Regina tells Hook about the dark curse, as she says "This new realm", her arm is hanging down by her side. In the next shot, her arm is bent. (00:03:45)
Continuity mistake: At Lake Nostos, in the first shot of Emma, Snow and Mulan, there is a giant rock on their left. None of them move, but two shots later, the rock is on their right.
Continuity mistake: At Lake Nostos, when Emma says "You're not going anywhere. This portal's taking us home", in the wideshot, Snow's quiver strap is over her pink cardigan. In the close-up, it is under the cardigan. In addition, Mulan's and Snow's weapons are held at different angles between shots.
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.