Ki Sense, also called Reading Ki, is an ability that allows the user to sense ki and power levels. It is used by a vast majority of the Dragon Team, and was first introduced in the King Piccolo Saga of the Dragon Ball anime.
Ki sensing is the ability to sense the location, life force, and power level of anyone; the stronger and closer the enemy, the more powerful the sensation. Also, if a ki is very powerful, it can be sensed from afar by people who are trained to sense energy; like Gohan sensing Super Saiyan 3 Goku's energy all the way from the Sacred World of the Kai. Once a fighter masters the skill, they can use it even when their other five senses are incapacitated.
No outsider knows the sensations associated with ki sensing, as Mr. Popo likens explaining it to someone as being like "describing sight to the blind." However, the manga and anime usually show a minor spark on their head to signify that the Z Fighter is picking up a power.
Despite how common the technique is in later sagas, and how easy it is to learn for an adept martial artist, it actually appears to be a rare ability, as the Frieza Force was unaware of the technique until they faced the Dragon Team. Frieza and his army, who did not possess this ability, use a device called a scouter that can give numerical readings of a person's location and power level. However, scouters have been shown to be very fragile, breaking when a fighter powers up too much, making the ki sensing technique preferred for those who know how to use it.
Androids have an artificial source of ki and cannot be found by the ki sensing ability. The Dragon Team had to find them using more practical means. Some organic being also seem to have ki of a different kind which cannot be detected by this ability, as all three members of the Trio of Danger had ki that could not be detected even by Goku's godly ki sense.
Several more advanced versions of the technique exist, although all are treated as higher forms of the same base ability, allowing for more information to be available to the user than simply the current battle power of their opponent.
Namekians are able to sense whether the life force is good or evil; such as when the Ginyu Force arrived on Namek: Krillin senses their presence and thinks it is Goku, but Nail says "I, too, sense the presence of evil coming." The Namekian ki sense is displayed for the first time on-screen when Raditz approaches Piccolo on Earth: Piccolo reasons that the incoming power could not be Goku because "It's too horrible." Trained fighters also have the ability to sense whether or not a presence is good or evil, as Goku says "It's a horrible power, too! I'm trembling!" as he senses Raditz approaching Kame House, and later when he says that he senses evil when Black Smoke Shenron is summoned. Trained warriors like the Gods of Destruction, can read an individuals spirit in their ki for any evil intent such as the intention to murder someone. It is also possible for a skilled user to detect strength which is being concealed by the target. Krillin was able to detect that Cell was concealing his true power after being kicked by him, and Korin was able to approximate Cell's true power from a distance.
It appears as if each organism has a distinct ki signature, much like fingerprints in the real world. This is evidenced during Cell's introduction in the series, where the Z Fighters, prior to finding out who Cell was, described his ki signature as a combination of Goku's, Vegeta's, Piccolo's, Frieza's, and King Cold's ki signatures (the main components of Cell's biological makeup). This is backed up by Beerus and Whis revealing that Goku Black shares his ki signature with Zamasu. As it is later revealed that Goku Black is Zamasu after having stolen Goku's body, it is assumed that ki is linked to the soul.
Ki sensing is an invaluable ability that allows a fighter to detect the location of an opponent that is too fast to see, or is hidden. It can even be used to detect invisible beings, as evidenced by Piccolo using his keen ki sense to detect and knock off Gamisaras. However, the limitations of relying on the ability include being unable to detect certain beings such as Androids, and being deceived by beings such as Damon. Piccolo assumed that Damon was invisible when in fact he was too short to see. He was later detected using sound by Android 17. Some being such as Hit and Obni are able to project fake Ki which fools an opponent into thinking they are in a different place, whilst suppressing their own Ki. This effect is often accompanied by a visual illusion.
Gods have a holy kind of ki that cannot be sensed by Earthlings nor Saiyans; such ki can also be obtained by becoming a Super Saiyan God. It is possible, however, for a rapid change in the strength or nature of a person to render their ki unfamiliar even to allies. This notably happens as Gohan returns to Earth after training with Old Kai; Piccolo, Goten and Trunks are unable to determine who Gohan was until visually identifying him.
The Instant Transmission technique requires the user to home in on a ki signature before teleporting them to that ki signatures location or somewhere nearby. Meaning the ability to sense ki is a requirement (the Kai Kai technique however does not require a ki signature or the ability to sense ki).
- Godly Ki Sense - Since godly ki cannot be sensed by regular beings, gaining the ability to sense godly ki must be learned separately. Deities like the Gods of Destruction, their attendants, and the Supreme Kais can sense godly ki thanks to possessing it, and other lesser deities such as Guardians and Kais are also able to sense it. Goku gained the ability to sense godly ki after temporarily becoming a Super Saiyan God, and Vegeta gained it after several months of training with Whis. In Dragon Ball Super, Frieza gained the ability to sense Godly Ki after his training following his first resurrection as he could sense Goku's power in Super Saiyan Blue. Hit had the ability to sense Godly Ki, as he could sense Goku's power multiplying upon using Super Saiyan Blue Kaio-ken. After acquiring Super Saiyan Rage, Future Trunks was able to sense Godly Ki as he could sense Vegito's power in Super Saiyan Blue. Jiren was able to sense Godly Ki as well, as he noticed Top powering up to his God of Destruction Mode. By the time of Dragon Ball Super: Broly, Piccolo has learned how to sense Godly ki.
- Scout-Scopes - Non-wearable, computers used by the Saiyans until they were annexed by the Frieza Force.
- Scouters - These high-tech devices originally designed by the Tuffles and later modified by Frieza Force engineer Gichamu to artificially detect ki and calculate an opponents Power level. Due to the ability to sense ki being an extremely rare ability among members of Frieza's army, its members and allied races such as the Saiyans often rely on Scouters as an alternative mean for them to detect ki. However, like Ki Sense, they are unable to detect godly ki due to it being different from normal ki. However, unlike Ki Sense, Scouters can be destroyed and/or damaged removing their ability to detect ki until it is either repaired or replaced.
- 3D Scan Mode - Certain Androids, cyborgs, and robots come equipped with a built in feature that allows them to scan their opponents and even detect ki in order to determine their power level. Like Scouters, it is a way to artificially detect ki, though more advanced and serves as a way for entirely artificial beings to sense ki.
- Energy Meter - A handheld device used by Babidi and his minions designed to calculate power levels far too high to measure even by using the best Scouters, using a different form of measurement known as Kili.
- Similar to the Frieza Force's dependence on scouters, the Dragon Team were shown to be somewhat dependent on Ki Sense in the Majin Buu Saga, as they assumed Gohan was dead simply because they couldn't feel his energy (in spite of having used the Dragon Ball to resurrect people shortly before), which made them unable to realize that he was actually on another planet. Videl even points out this by stating that they didn't even found Gohan's body.
- This is somewhat ironic, as the Z Fighters were enemies of the Frieza Force.