The Mind's Eye of the Kagura classifies Karin as a sensor.
A Sensor (感知タイプ, Kanchi taipu, literally meaning: Perception type) 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.[1][2] 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.
Throughout the series, various characters have commented on the 'feel' of a person's chakra and similiar 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.
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, 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.
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.