Everyone has friends who tell stories of seeing a Fairy, and everyone's cousin claims to have had one light on their hand.
In reality, it is hard to actually see a Fairy. Fairies will never just appear in front of you. Usually they can be seen as a glint or a flicker in the corner of the eye. The little motes in a sunbeam are Fairies, but the closer you look, the harder they are to see. Maybe you sense someone else near you, but when you turn you only see a vague outline of where they were.
Fairies take all forms from microscopic to large, but are always humanoid and always nearly translucent. When you see a bright sunbeam with little glitters in it, or catch a glint of light off the morning dew, those are fairies dancing in the sunbeam.
There is great debate as to whether Fairies are actually aware of us or our plane or if they have any intelligence at all. There have been no records of any Fairy being seen holding or wearing any manufactured item, and their movements while deliberate do not seem to be guided. It is not known if they are aware of the real world or even if they are aware of other Fairies.
Fairies come and go between the planes at will but seem to be nearly completely oblivious to our world with one exception: they are drawn to beautiful things like gems, flowers, sunlight, art... Peaceful meadows in deep forests often teem with Fairies dancing in the sunbeams and among the flowers. Intricate gems will glitter with the effects of Fairies flitting in and out of existence near it.
Fairies can be found near all beautiful things, perhaps almost reaching out to them or dancing around them. And while they seem to avoid physical things in our world, it could be something more like how water flows around rocks not because the water knows to avoid the rocks, but more because that is just its nature.
While Fairies avoid physical things in our world, they can and will pass through them. They are more likely to move through non-living things than living things, and more likely to move through non-magic things than enchanted things.
When a Fairy visits a place in our world. It leaves behind a tiny burst of Magic that will spread out and infuse the area. While it is not a physical thing, people often use the term "Fairy Dust" to describe the effect. Like a residue of the presence of Fairies.
Fairies can not be interacted with my any known magic or physical means. No device can see a Fairy. From magnifying glasses to the most carefully crafted magical recorder. Magical traps designed to detect movement or the presence of other magical effects will never trigger for a Fairy. The one exception to Fairy detection are Fairy Cages.
Despite the name, Fairy Cages can not catch Fairies.
Fairy Cages are intricately crafted wire sculptures that are sensitive to the presence of Fairies. In the presence of Fairies, the Cages will twirl or turn similar to a small wind driven sculpture and often care has to be taken to tell if the Cage is moving because of Fairies or because of a breeze.
Cages are usually made of silver or copper wire occasionally with gold highlights or dangles. Designs vary from utilitarian to intricate.
Usually no bigger than a small bowl, most can be held in the hand or suspended from poles or ceiling fixtures. The tiniest are made into necklaces, the largest hang in the Elven halls and are studied by Elven mages for any hint to the nature of Magic.
Crafting a Fairy Cage is far more of an art than a science and the exact same apparent design may work for one Cage and fail in another. Crafting a Cage can take anywhere from an afternoon to several weeks and an established crafter can draw commissions from a wide area for their better works.
As a general rule, cage crafters are not themselves magic users. Possibly the ability to interact with magic interferes with the creation of a fairy cage.
Small, simple Cages are common and many houses will have one on a window ledge gently turning in a sunbeam more as a decoration than a useful detector. These cheap Cages are "mass produced" and are rarely sensitive to all but the more obvious Fairy sites.
More sensitive detectors are much rarer and much more expensive. There are rumors of Elven Cages that can detect an increase in Fairies before they even happen.
Mages, clerics, or other magic attuned individuals will always have at least one cage and sometimes several tuned to specific functions. Some mages will embark on years long quests to find an improved cage.