The Sight is a psychic ability that occurs naturally in some people. It allows them to see things as they really are, revealing fey and other spirits that are trying to hide, as well as see through Glamour.
Animals may also possess this ability. Does your dog or cat react to things you cannot see? Then there is something there.
Some entities may react badly to a person with The Sight. They do not like being spied upon.
In a tale about a mortal girl hired as a governess to a fey household, the parents instruct her to apply an ointment to the eyes of the child but to keep it away from her own. Curiosity overtakes her, and she reasons that she must see that it is safe for the baby, she must test it on herself. She applies it to one of her eyes, and the result stuns her. Through the anointed eye, she sees the rich house she has been living in is actually a hovel, and the beautiful parents and baby are actually quite ugly. The parents see a change in her behavior and she admits to using the ointment. They ask her which eye she can see them with. When she answers, they blind her in that eye.
The Faerie Ointment on the eyes of half-fey children allows them can see the world as it is and interact properly with other fey. The effect is permanent.
A Seeing Stone occurs naturally when water has drilled a hole through a stone. Looking through the hole allows the user access to The Sight. Seeing Stones can also be made artificially by drilling through enchanted rock.
A sacred plant, the Shamrock is able to break fey magic. So it is possible to see hidden faeries and through their glamour when you have one in your possession.