Sensor Type

A is a ninja that is capable of detecting the presence of other people by directly sensing their chakra.

Overview
A skilled sensor can detect their targets from a great distance, as well as differentiate the characters by their chakra signature alone. They are even able to tell when other sensors are using their abilities. Karin is able to tell exactly who is among a crowd of people and her sensory skills are such that she is even able to tell if her targets are lying based on any fluctuation of their chakra. Mū and Tobirama Senju can discern between chakra signatures of shinobi from different populations. Nagato, another skilled sensor, was able to detect the exact location of the person controlling them despite that person erecting a barrier around themselves. Members of the Yamanaka clan have even displayed a technique whereby sensory perception can be transferred to other non-sensors.

There are, however, methods that can hinder a sensors ability to detect individuals. Mū has mastered the Dustless Bewildering Cover to such extent that he is able avoid detection by traditional sensory techniques, but can be sensed through other means such as contact type sensory techniques. Karin is able to suppress her chakra though she herself cannot use her abilities while doing this. The Aburame clan members can use their insects to create false chakra signals and confuse sensors. The Demonic Illusion: Steaming Multistoried Building used by the Second Mizukage and his Giant Clam also prevents them from being sensed through traditional methods while in effect. Zetsu is also able — through unknown means — to render himself undetectable.

Throughout the series, various characters have commented on the "feel" of a person's chakra and similar comments regarding the ability to sense high levels of chakra a little, leading to the possibility that chakra sensing is merely a more refined version of this by either teaching or being simply born more sensitive. Several other methods of "sensing" have also been displayed such as Gaara's Concealed Sand Picture Cat technique, which is a sensory ability as well as Kidōmaru's Spider Web Area and Nagato's Rain Tiger at Will Technique.

According to Karin in the anime, sensor types have different specialities depending on their villages: Konoha shinobi often use ninken with their smell and tracking sense. Karin's sensing abilities are derived from her Mind's Eye of the Kagura; it is unknown if other sensors also have a specific techniques to do so, or if it is just a general skill.

There are also other methods of sensing, Tobirama Senju and Minato Namikaze were able to detect enemy presence and numbers by simply touching the ground with their index finger. Tobirama went as far as being able to tell that Orochimaru's new body was made almost entirely of Hashirama's DNA. Gaara also possess a sensory ability which is tactile in nature: Concealed Sand Picture Cat, which allows him to locate targets that his sand comes into contact with, even if they are invisible.

Sensor types can also detect changes in a person's personality by sensing their chakra as chakra reflects the user's personality.

To be able to sense an opponent, a shinobi must have their chakra to "sensory mode". As such, if they are distracted and don't have their chakra on sensory mode, they can be kept from detecting enemies.

Trivia

 * Sages, who utilise natural energy, can also detect presence of people through their chakra, essentially making them sensors: in fact, the word uses the same hiragana: せんじゅつ, as the art of, an allusion to the abilities of Sages to sense persons while in Sage Mode. Also, Nagato was able to reverse track Kabuto when he emitted his chakra through their control tags of the Impure World Reincarnation and Naruto was able to do the same thing to Nagato with the chakra stakes.
 * After gaining control of Kurama's chakra, Naruto has displayed the fox's ability to sense others' presence by detecting negative emotions, an ability that Kisame stated other sensors cannot do.
 * Naruto can combine both Negative Emotions Sensing and Sage Mode chakra sensing using his senjutsu-enhanced Tailed Beast Mode and the Six Paths Sage Mode. Upon achieving Six Paths Sage Mode, Naruto's sensory capabilities were empowered to the highest possible level.
 * Kisame Hoshigaki's blade, the Samehada, is attracted to intense chakra, allowing him to hunt down such individuals.
 * It should be noted that while in the anime, Karin referred to persons using ninken as sensors, this was not said in the manga.
 * Sasuke claimed that Karin's ability to sense chakra is unlike that of anybody else.
 * In Naruto: Shippūden episode 226, the sensor type shinobi that was on the ship exhibited the ability to sense things that would generally fall out of the realm of sensors, such as being able to tell that they were far away from the depth charges being fired, seemingly sense air currents underwater as well as tell that there was a large subterranean hot spring underneath them.