Sensor Type

A is a type of ninja who is capable of detecting the presence of other people through their chakra. A skilled sensor can detect their targets from a great distance, as well as differentiate the characters by their signature chakra alone. Such examples include Karin being able to tell exactly who is among a crowd of people, and Fū being able to tell that a sensor type is within a group. Karin is good enough to tell if her targets are lying based on any fluctuations of their chakra. Other sensors are also able to tell when this technique is being used.

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.

According to Karin, in the anime, Sensor types vary between the Hidden Villages: i.e. the Ninken of the Inuzuka clan can smell chakra, the Kikaichū of the Aburame clan can home in on sources of chakra as well as disrupt chakra readings in an area, and the Byakugan of the Hyūga clan can see and read chakra from varying distances are all examples of Sensor ninja who are allied to just Konohagakure. Karin's sensing abilities are derived from her Mind's Eye of the Kagura; it is unknown if other sensors also have a specific jutsu to do so, or if it is a general skill.

Trivia

 * Sages, who utilize natural energy, can also detect presence of people through their chakra, making them somewhat like 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, something that no other sensors can do.