|Francesco Goya obviously had witch-sight.|
They are normally invisible except to those humans who have been granted "witch-sight" through sorcery. However, their deeds sometimes manifest as omens, portents and misfortune for humans.Aw, man. Wouldn't be a gorgeous bit of color for, say, Beyond the Wall? Or any folkloric campaign?
(As an aside, this would be a great campaign rationale. Ever wonder why the king with his retinue of knights can't go after those goblins threatening the village? They need the adventurers, witch-sighted all, to actually see the goblins.)
Everybody knows that there are beasties lurking about, and blame them for everything that goes wrong. Sometimes they're even correct. Everyone has a story about seeing a sprite when they were a child, or a kelpie in the swimming hole, and obviously that barn didn't catch fire by itself, there must have been a dragon! But the PCs were born with a caul over their eyes, or under a shooting star, or something. They are the real deal.
If you have non-human characters, and you didn't want them to be invisible bystanders during all in-town interactions, you might need a complimentary trait for them,where they can appear to humans. Probably in a human guise. Dwarves would appear to be short humans, elves as pretty youths, and satyrs seem unusually hirsute.
Maybe the visible fey are changelings who were raised by humans. Or they're wearing a glamour. Or they are somehow tied to the earth that brings them closer to humans, but causes them to be ostracized by their own people.