Fey characters can do so naturally. For them, it's like looking at an "ambiguous image". Like the famous Duck Bunny. You can force yourself to look at it one way, or the other.
For Human characters, we shall assume they all have some talisman, artifact, charm or other power, maybe just an inherent "Gift" that allows them to see things as they really are.
It wouldn't help for characters committed to work with the Faerie world to be unable to see it.