In any setting where there are people with uncommon powers, be they magical, psychic, or ki-based, it's a common trait for those blessed with such abilities to be able to sense it in others. This often results in simply having characters remark of one another: The Force Is Strong with This One. But sometimes, they sense more than the sheer amount of power and can actually detect fine details in what's going on. There are two basic ways this works:
Activity DetectionCharacters can sense when others are using magic, and perhaps exactly what they're doing. Often tied to vision or hearing, but could also just be some sixth sense unrelated to those possessed of normal people. The signs of magical use are proportionate with distance, and by the degree of power being emitted. There are probably tricks to avoid detection or damp the signs of use, and other tricks to see through those.
Residual DetectionUse of magic leaves signs which can be sensed by those with the right gift. Any place where magic has been performed, and any object or person under the effect of a spell will have a residue that fades with time. The amount of detail which can be gleaned from this method varies. This is common in settings which have a Background Magic Field.
The sensitivity of magic-users to one another is important in the development of a magic system. It acts as a limiter on the more powerful users in High Fantasy: The mighty wizard could wipe his current foe off the map with ease, but doing so would attract the attention of much darker and stronger forces. Thus, he has to resort to more subtle and indirect magics, and save his strongest stuff for true emergencies. Sensitivity also plays a key role in unraveling a plot, as the characters check residues and enchantments to try and uncover who did what long after the fact.
Compare Magic Is a Monster Magnet, where possessing or activating powers attracts the wrong sort of attention.
- Bleach: Powerful spirit creatures such as Soul Reapers and hollows can detect the use of spiritual pressure (spirit energy) a considerable distance away.
- Fighters in Dragon Ball Z can detect the use of ki from others, as well as potential, though that can also be concealed. Freeza's army cannot, but they have special sensors for that. They also apparently had never encountered anyone capable of either feat, because Vegeta and Nappa are surprised by it; Vegeta eventually destroys his sensor, since it's useless against those foes and he's starting to learn how to sense ki, anyways. Later, during the series' stint in Namek, he keeps perfecting that technique. He detects Krillin and Gohan, who are hiding a dragon ball from him, but a whale-like thing jumps up nearby and he reasons he must have detected that. He also starts learning how to conceal his ki, keeping it nearby his former power level, to get the drop on the rest of the army and the generals.
- Detecting the aura of Youkai is a useful ability in InuYasha, common to Youkai themselves, Hanyo like Inuyasha, and to ki using humans like Kagome and Miroku. Even exterminators like Sango could sense it to a lesser extent. There was an old exorcist who was completely insensitive to youki, which turn out to be rather helpful since it didn't affect her either.
- In That Time I Got Reincarnated as a Slime, all monsters have Magic Sense, which detects nearby sources of magic, gauges Power Levels in individuals, and can be used to communicate telepathically.
- In Yozakura Quartet, half-youkai can sense youkai energies, including in one another. "Uncle" Jinroku can also sense the dragon pulse in the earth to help others fight fallen youkai and to find good clay and bamboo.
- Fighters in Yu Yu Hakusho usually can only tell how much energy is being put out in a given direction and by whom, but in the Dark Tournament arc Kurama and Hiei are able to sense Genkai's death.
- In Powers of Invisibility, Tikki and Plagg explains that the fact that Juleka was unaffected by the Glamour protecting Marinette and Adriens secret identities was because she is a candidate to take on the role of the Guardian and wielder of the Turtle Miraculous. She then sarcastically refers to this as her "turtle sense."
- Sorcery in The Belgariad and related works makes what is described as a "sound", which can be mistaken for mundane noise under some circumstances. The sound can be lessened by doing things slowly, and shapeshifting, because it's directed internally, is very quiet. The protagonists are often forced to avoid sorcery in enemy territory because of this noise effect. Interestingly, other forms of supernatural power, such as magic, wizardry, necromancy, and prophecy, do not make this signature noise.
- In Katherine Kurtz's Deryni, mages work this way. They can generally tell who is Deryni and who isn't, because Deryni have mental/psionic barriers called "shields" that are detectable over short distances. Some highly skilled Deryni can disguise their shields, making them seem transparent. Ordinary humans lack these shields, so their surface thoughts are easily apparent to Deryni. Transfer Portals have a residual signature that Deryni can feel and recognize (described as a tingling sensation coming from the area of the floor/ground where the Portal is located).
- Magic in the Discworld falls under both types. Strong magic leaves strong residue, to the point that especially strong magic can leave magical fields behind that warp reality and last for centuries. All wizards, and only wizards, have the ability to see "the colour of magic" (octarine, an eighth color which is outside the normal 7-color spectrum), a color which appears whenever magic is being used.
- Wizards in The Dresden Files can use their True Sight to see exactly what is going on. The Sight shows everything that's happening, in excruciating and often maddening detail, and one of the things that can be Seen is the residue of magic that has been used recently. Magical practitioners can also detect each other when they touch, meaning a practitoner hiding that ability needs an excuse to avoid shaking hands.
- In The Elenium series, use of magic can be detected in much the same way, but the range of detection seems to be much less. It's also possible to tell which spell is being used by the feel of it. Notably, the Bhelliom leaves a mark with its passing that any god or similarly powerful being can't help but feel, and using its power draws the attention of all such beings. This necessitates the protagonists to keep it in a gold box most of the time, which nullifies the effect.
- Magic users in Fancy Apartments can sense magic. There's also second sight, which is basically a more accurate and specialized version.
- In Harry Potter and the Half-Blood Prince it is suggested that powerful wizards who actually know what they're doing are capable of the residual type; in the cave scene Dumbledore is shown to determine where the secret entrance is and the spells used on it without the use of any detection spells.
- In The Film of the Book, they add to this with Harry being able to sense the residual dark magic indicating that an object is a horcrux, which is necessary as they trimmed out a long dialogue where Harry and Dumbledore discuss what and where the horcruxes are. It also works as foreshadowing that Harry himself is a horcrux.
- Incarnations of Immortality: In For Love of Evil, Parry is tracked through France by a sorcerer who can sense when he does magic.
- Randall Garrett's Lord Darcy series. A Witch-Smeller can detect the taint of black magic on its caster and its victims.
- Mage Life, The main character makes use of additional senses attributed to his capabilities with magic.
- Robert A. Heinlein's Magic, Inc.. Dr Worthington is a witch smeller, who detects and analyzes the magic forces that trashed Archie's store.
- Played with in the Mistborn series: a Misting who can burn bronze (known as a "Seeker") can detect Allomancy, but a Misting burning copper (a "Smoker") can hide it.
- It's later revealed that very powerful Mistborn, like Vin, can even detect Allomancy through Copperclouds. Except even that is later revealed to be the result of a bronze Misting or full Mistborn getting another bronze Misting's power added to theirs through Hemalurgy.
- Linden in The Second Chronicles of Thomas Covenant has the empathic ability to sense when things in the Land are healthy or diseased. Too bad that Every. Single. Thing. in the Land is corrupted and poisoned by the Sunbane.
- Terran psychics in the Expanded Universe of StarCraft can detect each other. Even some weak ones without otherwise useful powers can do so. Many are hired as "wranglers", who hunt down psychic children and forcibly recruit them for the Ghost Program.
- Star Wars Legends: Luke Skywalker has Activity Detection just fine, but in The Courtship of Princess Leia he is conspicuously unable to tell if random people are Force-Sensitive.
- In Tamora Pierce's Tortall Universe, characters with the Sight can observe use of the Gift. Alanna also has a stone which, when held, allows her to see magic in use.
- Channelers in The Wheel of Time series can always tell if someone of the same sex is channeling nearby, and how strongly, but need to see the flows of Power in order to determine the nature of the weave used. Both the ability to channel and the weaves themselves can be concealed, by methods long-lost but rediscovered partway through the story. Weaves also leave a residue that can be read by other channelers of the same sex (though the ability is uncommon), the most common use being to follow someone who Traveled away.
- Among their array of unnerving powers, Myrddraal can sense when someone is channeling nearby. This ability is nowhere near as sophisticated as that of a real channeler; one of them describes it as an "itch."
- Gholam, designed to be assassins of channelers, seem to be able to sense the force and direction of channeling.
- Female channelers can sense the ability to channel in each other, even if neither is actually touching the True Source at the time, although they do have to be quite close. Once they're touching the True Source they literally light up like beacons - another female channeler will, when sufficiently experienced, be able to see a glow around a woman who's holding or using the Power. However, they are completely unable to sense when a man is channeling or holding the Power, at least until his spell takes effect and leaves a residue.
- Men, on the other hand, can't identify another man who can channel unless he's holding the Power, and even their sense of that is much less precise than the sense women have of each other. This is probably related to the role of female channelers as the catalyst for linking strength - men cannot link without women involved, but women can link without men. However, men (or at least, Rand) can sense when a woman nearby is holding the Power, although to them it generally feels like nothing more than a vague sense of unease.
- Blue Rose adepts, regardless of specialization, all have at least some talent at sensing arcane occurrences and energies. Non-adepts can either specifically train in the Second Sight, or get magic stones that glow in the presence of arcana.
- The spell Detect Magic in Dungeons & Dragons lets magic-users see magical auras. Sufficiently long study (and a sufficiently high roll) allows the spellcaster to determine what school of magic the magic is from, and what the magic (or magical item) does.
- In addition some monsters had the innate ability to detect magic (as the spell), such as several types of demons and devils, efreeti, imps, gynosphinxes and so on.
- Several psionic monsters had the ability to detect the use of psionics nearby, such as the Brain Mole, Intellect Devourer and Thought Eater.
- In 4th edition anyone with training in the arcana skill can detect magic at will.
- In In Nomine characters with supernatural powers (mostly angels and demons, but some special humans as well) can detect "Symphonic Disturbances" caused by use of certain supernatural powers, and/or controlled Essence expenditure nearby.
- Characters in the New World of Darkness can purchase the Unseen Sense merit, which will let them detect one particular sort of supernatural activity. A character with Unseen Sense (Vampires) would be able to pick up when a vampire uses a supernatural ability nearby, but could not pick up on someone simply being a vampire. Mages get a form of Unseen Sense that picks up all supernatural activity and get the "residual detection" form of this trope from scrutinizing the resonance of something through various Mage Sight spells.
- Characters in Reign can learn the Eerie skill, tied to the Sense stat, to detect the use and presence of magic. Anyone has access to Eerie whether or not they can perform magic, and can roll just their Sense stat even if they're completely untrained in the skill. It's a natural sixth sense that everyone has, even if some neglect to pay attention to it.
- Shadowrun. Anyone with the ability to perceive astral space (including magicians) can "see" magic use, including spells. Awakened creatures with the Magic Sense ability can detect the presence of magic, including Awakened characters (e.g. magicians) and other Awakened creatures.
- In Devil Survivor, the protagonists eventually become able to sense the proximity of wild demons after gaining experience in fighting them off.
- In Dragon Age, the Fade is a metaphysical realm existing alongside Thedas, that all races (barring Dwarves) enter when they dream. Since Mages derive their powers from their strong connection to the Fade, they are uniquely capable of sensing locations where the Veil seperating the two worlds has become weak.
- Every tainted being (Grey Wardens, darkspawns, ghouls) can sense the taint in others beings.
- Templars can sense magic and the presence of demons.
- Persona-users in the Persona 2 duology can sense a "resonance" when they get close to other persona-users. The idea doesn't really come up in other Persona games except with Fuuka (in Persona 3) and Rise (in Persona 4), who both play Mission Control.
- In the StarCraft universe, some weaker psychics get jobs as "wranglers", using their powers to locate other psychics. Zerg also have this ability. The ability of other psychics to detect psionics depends on who is writing the novel.
- In Tales of Phantasia and Tales of Symphonia, people with elven blood can sense the flow of mana, allowing them to recognize when magic and magitechnology are being used, measure the world's background mana, and distinguish the mana signatures of elven-hybrids from those of pure-blooded humans.
- Mages from Fate/stay night can sense the use of prana in various ways. Emiya Shirou is particularly sensitive, and detects magecraft as a scent.
- Raven from El Goonish Shive can "taste" magic through his ears, and analyze the nature and power of the spell by the "flavor" of the magic, because he is half-Immortal. More recently, Tedd Verres has been revealed to have an ability to see magic spells being cast, and tell details about the spell itself. Also, Luke has the ability to see who has magical ability, although not what that ability is.
- Sette has the unique ability to smell magic, though not well enough to stop her from walking right into a gossamer snare trap.
- Skilled spellwrights can feel fluctuations in the Background Magic Field of the Khert when there's unusual magical activity in the area. The Platinum caste have the strongest connection to the Khert and are the most sensitive — the disturbance from some dangerous Magitek leaves Quigley itching uncontrollably and slurring his words.
- Jack O'Malley from Widdershins can see people's auras and invisible (but not intangible) spirits that were improperly summoned. He hates it because crowds and wizards hurt to look at, and it's left him completely colorblind except to those auras and spirits.