I don’t think Emma’s lie detecting ability is actually a super power. I always thought that was just a joke to get Henry onside.
Given her past life, pre storybrooke, I think she would need to “read” people well. In certain lines of work, you get pretty good at reading people, picking up on non-verbal cues that give them away. In my job (criminal law), we call it our radar or antenna. I can’t tell if my client is lying to me, but I can get a reliable sense of when I need to ask more questions or something is off. I think that’s all Emma’s “superpower” is – not a magical ability to tell if someone is lying but just a skill from her past to read someone and tell when something is off.
And in that case…. Damn. She should have read through Greg. His non verbals were screaming “I saw magic but I’m not telling you in case you chuck me in the psych ward”