Grand Moff Tarkin: Obi-Wan Kenobi? What makes you think so?
Darth Vader: A tremor in the Force. The last time I felt it was in the presence of my old master.
Suppose magic, ki, or Psychic Powers exist, and that there are people who possess these powers. Naturally, how strong a given person is with these abilities will be a big factor in the plot, both in the ass kicking scale and in terms of how characters act and react to each other. But how can writers convey that to the audience? Sure, generally the rule is "show, don't tell", but what if a given character's psychic powers are to pinch shut the carotid artery? Or their Supernatural Martial Arts can crack a planet in two? Having them remind viewers "Luckily, My Powers Will Protect Me" gets old fast. And you can't very well have them show the power off every time they appear so new viewers can get clued in, just imagine what that'd do to the Special Effects budget!
So instead, writers will give these characters the secondary (or primary) ability to sense each other's power, much like internal Stat-O-Vision that is represented with conveniently color-coded Aura Vision. Typically, The Empath or someone with a Significance Sense or Spider-Sense will be able to tell the raw Power Level of an opponent, what brand of magic they use (especially if they're evil) and the like. Expect them to intone, "The Force is strong with this one," or a variation when a powerful enemy draws near; especially common is for the sensor to intone "Such oppressive evil power!" before fainting or falling to his knees in fear.
This can result in one being a Weirdness Magnet.
It's practically a requirement for any Shōnen Anime or Manga that at least one person can do this (often a Mentor and someone else in the hero's group to take over when the Mentor Occupational Hazard kicks in), especially right before a climactic battle. Western media may use Cower Power, a Game Face or other methods to show this off.
Compare Killing Intent, which can have similar applications but is more about danger level than power level, Supernatural Sensitivity, and Psychic Radar. When one's Strength with the Force is pretty much a guarantee of victory in a fight, it's My Kung Fu Is Stronger Than Yours.
- In Black Clover, mana is an energy that flows in (almost) every living being. Advanced magic users can detect the general strength of another user by sensing how much mana they possess. Ki also works similarly.
- In Bleach, Soul Reapers (and other spirit creatures such as Hollows) can detect the level of spiritual pressure (Power Level) another creature has. Sufficiently great differences between the sensee and the sensor can invoke side effects such as weakness, paralysis, and fear in the sensor There are cases where an individual is so powerful that their Spiritual Pressure cannot be sensed at all, like Aizen in his final stage of evolution. This baffled everyone.
- Claymore: Yoma, Claymores, and Awakened beings can sense each other's presence and the amount of yoki. However, some of them can mask their aura to appear far weaker than they are (Riful) or even completely suppress it so only direct contact with their body will reveal that they are anything other than human (Priscilla). The Claymores have a pill with a similar effect. This is the only reason anyone survived the Northern Campaign.
- In Code Geass, it appears that those who possess a Geass Code or are in the Sword of Akasha can sense Geass users.
- Dragon Ball Z. Lots of times. Notably, all the races affiliated with Freeza do not know of this ability or how to use it, and instead rely on mechanical devices which prove largely unreliable since the protagonists' Power Levels fluctuate naturally and can be consciously pushed up or down. Once the Power Level numbers start getting out of hand, the series goes back to using this full-time. This proves to be a drawback in the Android Saga, since the mechanical / cyborg villains are undetectable in this way. In the Buu saga, Gohan freaks out when he senses Buu's power for the first time. In the English dub, he describes Buu's power as a bottomless well of power that still somehow felt empty.
- This kind of sensitivity is actually treated as rare when introduced in Dragon Ball, a technique known only to gods and requiring really complex training to master. It would be a borderline Story-Breaker Power when Goku learned it, due to rendering all forms of subterfuge futile, if his current nemesis Piccolo Jr. didn't happen to also have the ability. However, by the time Z comes along, it's So Last Season and every major character from Earth can do it, while Vegeta figured out how to do it off-screen within a few days of finding out it was possible. The next big trick is learning to suppress one's ki to make an enemy overlook or underestimate it.
- Elfen Lied of all things has this; Diclonii can sense each other's presence out to a certain undisclosed range, accurately enough to positively ID the other diclonius through this sense alone if they've met in person before. Well, mostly. Strangely, Nana cannot sense Nyuu, but when Nyuu pops back into Lucy, Nana IMMEDIATELY picks up on it, despite being a good distance away.
- It actually makes sense if you consider Nyu a depowered state of Lucy.
- In the manga, the Unknown Man takes advantage of this in the most horrific way possible. He rigs up a limbless Diclonius girl into a backpack-sized contraption to use as a Diclonius detector. After having had his way with her, of course.
- In Fairy Tail, it's possible for mages to sense the relative magical power of their opponents during a battle, usually through the form of some sort of pressure or Battle Aura, though really anyone can do so if the mage in question isn't even bothering to hide it. Typically, the more a character stares in awe and/or fear as an enemy is powering up, the better you can gauge their relative strength compared to each other.
- In Fullmetal Alchemist, the Xingese characters can feel the flow of the planet's natural energy, which is useful for finding homunculi, who feel like several dozen people in one body.
- In Full Metal Panic!, a variation of this can be found with Sousuke. He's shown to have a sort of sixth sense, in that he actually has the ability to sense "killing intent" in the air, and can somehow know when there are enemies that want to kill him. He demonstrates this ability several times, though one of the more pronounced instances can be found in the later novels. Tired, wounded (having had a huge portion of his liver blown out of him by a shotgun), and confined to a hospital bed, he was shown to be able to sense a bunch of soldiers sent to kill him, and therefore managed to lay a trap for them by the time they arrived.
- Apparently, it only works for when people are out to kill him specifically. If this worked on people trying to kill someone else,the show wouldn't have most of its comedy.
- The fact that he's extremely paranoid (for good reason, due to his history) probably helps.
- One of the reasons he had a hard time at school was because an agent observing him would spy on him through the scope of her gun, setting off his sixth sense.
- Gundam has a very particular term for this concept: pressure. The Newtypes of Gundam all have the ability to both sense pressure and exude it, with one of the few gauges of a Newtype's strength being the intensity of the pressure they exude. The truly powerful Newtypes, like Paptimus Scirocco of Mobile Suit Zeta Gundam and Haman Kahn of Mobile Suit Gundam ZZ, exude such strong pressure that practically the entire cast senses it when one of them shows up.
- Hunter × Hunter has this with many of the characters being able to sense "Nen", especially with a technique called En, which has a radius in which Nen users can be sensed. Practically all characters are able to use this to some degree, although the most powerful and wide-ranged of these is that of the Chimera Ant King Meruem.
- In Inuyasha, monks and priestesses can find youkai in this way, and also realize how strong they are.
- In Magi: The Labyrinth of Magic, Yunan, Scheherezade, Matal Mogamett, and Ren Gyokuen immediately realized that a miracle occurred when The Chosen One and resident Big Good Sinbad was born.
- Several wizards try to sense Kobayashi's magic powers in Miss Kobayashi's Dragon Maid after she got first place on the written part of a mage exam. They end up sensing Tohru instead (who was sitting next to her), which causes Kobayashi to be Mistaken for Badass. All of the dragons can also be seen reacting when Tohru's father shows up in the anime.
- Most ninjas in Naruto can feel the chakra of others while fighting them up close, but the ability to detect and gauge it from a distance is very rare. Karin, Fu, Ao, and C are some examples. Karin has also shown to use it to become a Living Lie Detector and can hide her chakra from other sensors (but can't sense herself when doing this), but it's unknown if anyone else can. Most of them also showed the ability to tell when someone else was sensing their chakra.
- Sage Mode also grants its user the ability to sense chakra from a distance, likely because the power involves controlling the natural chakra around him. Somehow, after gaining this power Naruto is immediately able to identify individual people by their chakra from beyond visual range, despite (since this was the first time he ever used the ability) having no pre-existing frame of reference as to what each of his friends' chakra should "feel" like.
- Kisame's Samehada can apparently sense/smell especially large bodies of chakra. This is possibly a reference to shark's ability to find prey with electrolocation.
- Later when Naruto enters his new kyuubi form, he can sense negative emotions, thanks to nine-tails.
- The Hyuga clan with their Byakugan can do this see chakra.
- Akamaru can smell chakra.
- Some members of the Yamanaka clan can do this.
- Pain could detect a person's strength if it stepped into "his" rain.
- One Piece:
- Fukurou of CP-9 can do this, albeit in an unorthodox fashion. He hurls himself bodily at his teammates and receives a deflecting punch or kick. Then he stands up and rattles off their respective power levels based on the force of their attack.
- Color of Observation allows this as one of its primary effects. Blackbeard has more than once commented on Luffy's strong spirit, and Silvers Rayleigh can accurately detect every creature throughout an entire island (more than five hundred, for the record) that Luffy could not defeat at the time.
- In Pokémon: Lucario and the Mystery of Mew, Lucario's Aura Vision can see when someone has a particularly strong aura.
- Saki is a manga about Mahjong, and therefore, strong enough mahjong players can feel the auras of other strong mahjong players.
- In one chapter of the spin-off Saki Achiga-hen, the main characters (a different group of girls) pass Saki in a hallway at the competition venue. Their coach (said to have been the strongest player in the history of their school) and one of their most skilled players freeze up in terror as she gets closer, despite not even knowing who she is.
- In The Seven Deadly Sins, Hawk is able to sense the power levels of other beings after Merlin gives him Balor's Magical Eye, a magical earring that grants him such abilities. He is also able to divide the power levels into four distinct categories, such as Magic, Strength, Spirit, and Power, all four of which add up to obtain the total power level of an individual. Hawk serves as the main method of conveying power levels to the reader, and while other characters can sense relative power levels, Hawk is the only one able to assign numerical values to them, making his ability useful for raw comparisons.
- Soul Eater
- Anyone who has "soul perception" can thus see the souls of living people). In the anime, this is shown the viewer via a Color-Coded for Your Convenience (blue = natural, red =kishin, purple = witch or misc.) glowing ball of proportional size to their power (with "accessories" for different characters).
- There are degrees of soul perception. Some, like Stein, can pick up on individual characteristics and emotions, others can sense when people are lying and see through the soul protection skill of witches (BJ), or extend their 'sight' over large distances (Azusa's clairvoyance). Maka can detect souls over wide areas (though not to Azusa's level...yet), picks up on emotions, and is implied (the Baba Yaga arc might have confirmed it?) to be good enough to see through Soul Protect.
- Maka frequently comments on the soul of whoever she is fighting, and Gopher for one is not impressed with her "peeping". The one time she was shocked with the results was early on when she sensed a true witch's soul for the first time. Considering it was Medusa, she had every reason to be scared.
- Ryou from Strawberry Shake Sweet. If you're a lesbian, she can see it in your aura. Heck, she can even tell who you're gay for:
Ryou: I've sensed the aura of a woman who is in love with her same-sex sempai two levels above her in the company.Julia: Wow, that's... really specific.
- Various characters, usually with the help of Millennium Items, in Yu-Gi-Oh!.
- Judai gains this power in Season 4 of Yu-Gi-Oh! GX. His spirit partner Winged Kuriboh seems to possess it from the beginning.
- Parodied in Mob Psycho 100. Phony Psychic Reigen Arataka has an excellent track record of convincing real, very powerful espers that he's the real deal. Absolutely none of these espers can sense a lick of spiritual power from him, but since the insanely powerful but very gullible Mob buys this completely and even credits him for teaching him how to use his own powers, most espers conclude that Reigen's just so skilled and powerful as an esper that he can effortlessly mask his aura.
- DC Comics: Unlike Marvel below, there aren't too many example of this running around in DC, due to the lower number of telepaths they have in general. One of the few reliable DC force-sensers is Captain Comet, a mutant psychic who was generally the go-to for this when he joined Vril Dox's L.E.G.I.O.N. The stronger DC telepaths like Despero and the Martian Manhunter are presumably capable of it as well, though they generally opt for more direct applications of their psychic powers.
- Marvel Comics: Mid to high-tier telepaths in the Marvel Universe are capable of this, with the concept seen most often in the X-Men corner of the Marvel U which has several extremely high-tier psychics such as Professor X, Jean Grey, Nate Grey and Exodus. A fight between two of these characters in X-Men Legacy put out enough psychic energy that telepaths literally on the other side of the planet could sense it.
- In Star Wars Tales #17, "The Apprentice", the Sith Master Finn says this about the slave girl Marka who had just mind-tricked his apprentice into taking her with him. Finn wonders if the Force led them to the god-forsaken rock of a world they are on just so they could meet her.
- Dreaming of Sunshine features Shikako, who is a chakra sensor to the point that she can see through disguise jutsu of people she knows well, gauge an enemy's aproximate strength, and detect lies.
- Ultraman Lunaram can detect another ultra by contact, and Father Leo can sense power with his mind in Ultraman Moedari.
- Powers of Invisibility reveals that Juleka has a subconscious ability to sense Miraculous holders, allowing her to see through the glamour that normally causes people to ignore the inconsistencies that come up around them, explaining how she figured out Adrien and Marinette even before she first saw them transforming. This also allows her to sense potential holders, and makes her an ideal candidate for the Turtle Miraculous and the Guardian.
- Fate/Harem Antics: As the Dragon Witch, Avenger/Jeanne Alter can detect Saber because of her Dragon Ancestry and quickly deduces her true identity as Arturia Pendragon.
- The Red Dragon's Saber: Rias and her peerage detect that while Artoria doesn't have Holy nor Demonic power, she has a lot of magical energy in her body.
- Immortals in the Highlander series can detect one another, as well as nascent immortals when they're in close proximity.
- Star Wars:
- Vader senses Obi-Wan's presence aboard the Death Star via his resonating in the Force, and later names the trope when he's chasing Luke down the Death Star's trench and finding himself unable to get a solid lock.
- Decades earlier, Qui-Gon had sensed Anakin's potential.
- In The Empire Strikes Back, both Palpatine and Darth Vader sense Luke Skywalker's growing power as a disturbance in the Force.
- In Revenge of the Sith, Palpatine quotes this line when he dubs Anakin "Darth Vader" for the first time.
- In The Force Awakens, both Kylo Ren and Snoke sense the emergence of Rey's latent Force powers as an "awakening in the Force."
- X-Men: The Last Stand had Callisto who could detect other mutants and ascertain their relative "power-levels" on a 1-5 scale. Only one level 5 has ever existed, and when she tells Magneto that she just became aware of a "level 5", he immediately knows that she refers to Jean Grey's "Phoenix" personality.
- In the Dragonriders of Pern series, the dragonriders' dragons can sense the psychic potential which is needed to bond with dragons. This is how dragonriders find prospective candidates for Impression during Searches. In the short story "Ever the Twain", the sibling bond between fraternal twins Nian and Neru was so strong that it actually prevented the dragons from accurately gauging Neru's potential, which is why they initially snubbed him during the Search until Nian argued that he be given a chance to Impress. In the end, both twins proved worthy, with Nian becoming the rider of the newborn queen Quinth and Neru (now N'ru) becoming the rider of bronze Larinth.
- Wizards in Septimus Heap can feel somebody's else's Magykal powers, which makes Marcia feel Septimus's Magyk long before he's revealed. Jenna feels his Magyk too in the first book.
- Star Wars Legends:
- Lampshaded by Tyria Sarkin in Wraith Squadron. She once says of herself, "The Force is weak in this one." She's Force-Sensitive, the last of the Antarian Rangers from Toprawa, but her Force powers are weak, and she feels overwhelmed by what she has to live up to.
- In Galaxy of Fear, Tash Arranda can feel the Force in Darth Vader. When she meets Jerec she recognizes the same sort of feeling in him but thinks he's not as strong.
- Female channelers in The Wheel of Time can always sense the strength of fellow female channelers, and perceive a glow around females actively using magic. All channelers can sense their own gender's magic in use, and spells are possible to sense the other gender's. Males channelers get goosebumps when females close by are using magic. There are also techniques to hide both ability and active use.
- Telepaths in Babylon 5 can usually sense each others' proximity. Similarly, Lyta was able to tell that Sheridan had also been Touched by Vorlons, but she didn't clue into it until Sheridan said something that only Kosh would have said. The human Psi Corps actually uses a rating system of P1-12 to quantify how powerful a given telepath is. P13 or higher are hypothetical, or Lyta Alexander. It is unclear if other species have equivalent rating systems.
- Drusilla from Buffy the Vampire Slayer also has this ability. Later it is suggested that many empaths are proficient in this ability.
- On Star Trek: Deep Space Nine, certain Bajorans can sense the strength of someone's pagh by grabbing them in the ear.
- Dungeons & Dragons has the spell Wizard Sight, used to discern arcane and divine casters and appraise spellcasting potential, and see magical auras of objects instantly and certainly (unlike "detect magic" probe).
- The Detect advantage in GURPS can potentially do this.
- Promethean: The Created has a similar rule — "Azoth [the "Divine Fire" that gives Prometheans life] calls to Azoth." On the plus side, whenever two Prometheans are within a certain distance, they can feel their Azoth tugging towards each other. On the minus side, so can the Pandorans...
- Vampire: The Requiem has Blood Potency and The Beast. The higher a vampire's blood potency, the more blood they can store and spend, and the harder they are to affect with negative powers. As BP rises, The Beast gets stronger too, so every time a pair of vampires meet, their Beasts will "confront" each other, with the stronger of the two trying to incite an Unstoppable Rage in its host, and the weaker one to make its own run away screaming.
- Keyblade users in Kingdom Hearts can sense light and darkness of peoples hearts as well as sense the hearts of people familiar to them. Only Terra and Aqua make use of this regularly however as it requires quite a bit of training. Since Sora hasn't gotten official training he's only made use of it once so far. He could sense that "Ansem" was actually his best friend Riku but not until Kairi hinted to close his eyes and feel with his heart. Also, particularly in Kingdom Hearts: Chain of Memories, Riku is able to "smell" the darkness coming off of Organization XIII members, likely due to his own foray into using dark powers himself. Zexion has a similar ability, noting the resemblance of Riku's "scent" with that of "the Superior" which makes sense since Riku's still carrying around the remnant's of "Ansem's" power (Xehanort's Heartless) while "the Superior" is Xemnas (Xehanort's Nobody).
- The Lufia series' Spiritual Force, which both heroes and big bads lampshade whenever they meet. By the DS remake, detection and unleash of Spiritual Force can be done even by oldmen.
- In the beginning of Star Wars: The Force Unleashed, Darth Vader defeats Kento Marek. Vader tells Kento that he can sense someone far more powerful nearby and demands to know where Kento's master is hiding. Kento claims that Vader killed his master years ago. Vader is surprised when he discovers that the powerful Force-user he sensed was Kento's son Galen (the player character), a little boy.
- In Persona 2, Persona-users can sense each other through a passive power known as "resonance", produced when someone is preparing to summon their Persona. If someone wasn't expecting to fight, then their resonance can come out of nowhere, but if they're preparing for a fight or in the middle of one, their resonance can be noticed from across a building. A malevolent resonance is generally uncomfortable to be around, but a particularly nasty one can cause a Persona to get scared, and at its worst it can be hard to stay standing in the face of that terror.