And the dagger may not even be in this world to begin with.

[And that's not what they want to do in this instance to begin with - they want to find Gold, not kill him.]

How would someone find out if they were the Dark One? Is there somewhere they would go or look or a ritual or something?

[Magic is not her forte. She's just spitballing. She is also dubious of the True Love's Kiss option but she's not really one for fairy tales. In her experience, they rarely ever come true.]