Fairies can not be interacted with my any known magic or physical means. No device can see a Fairy - from physical magnifying glasses to the most carefully crafted magical recorder. Physical or 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, and expensive gold sculptures may not be any more sensitive than simple peasant style twists.
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.