Sensor Type

A is ninja that is capable of detecting the presence of other people through 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.

There are, however, methods that can hinder a sensors ability to detect individuals. Mū has mastered the Hiding with Camouflage Technique 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 can use their insects to create false chakra signals and confuse sensors. The Mirage Genjutsu 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 Sand Sensing 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.

Tobirama Senju was also a very gifted sensor ninja. By simply touching the ground with his index finger, he could detect the presence and number of enemy shinobi. He was also able to detect Madara Uchiha's chakra on the battlefield some ways off and was also able to determine how Orochimaru's new body was made almost entirely from Hashirama's cells.

Trivia

 * Sages, who utilise natural energy, can also detect presence of people through their chakra, essentially making them sensors: Infact, the word, Senjutsu, (仙術), uses the same Hiragana, (せんじゅつ, senjutsu), as the Japanese art of Scrying (占術), a nod towards the abilities of Sages as Sensors.
 * Kisame Hoshigaki's blade, the Samehada, is attracted to intense chakra, allowing him to hunt down such individuals.
 * After gaining control of the Nine-Tails' chakra, Naruto has displayed the Nine-Tails' ability to sense others' presence by detecting negative emotions, an ability Mito Uzumaki, a former jinchūriki of the Nine-Tails also had and something that Kisame stated other sensors cannot do.
 * 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 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.