From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Ki Sense, "sensing a power level", "power sensing", or "sensing ki" is a supportive technique used by many fighters.
King Piccolo Saga
Ki Sense was the ability to sense the location, life force, and battle power of anyone; the stronger and closer the enemy, the more powerful the sensation. Once a fighter mastered the skill, they could use it even when their other five senses were incapacitated.
No outsider knew the sensations associated with Ki Sense as Mr. Popo liked explaining it to someone as being like "describing sight to the blind." The manga and anime, however, usually showed a minor spark on their head to signify that the Z Warrior was picking up a sense.
Freeza and his army, who did not possess this ability, used a device called a scouter that could give numerical readings of a person's location and battle power. Scouters had been shown to be very fragile, however, breaking when a fighter powered up too fast, making the Ki Sense technique preferred for those who knew how to use it.
Androids had an artificial source of Ki, and, therefore, could not be found by the Ki Sense ability. The Z Warriors had to find them using their eyes.
Namekians were able to sense if a life force was good or evil, such as when the Ginyu Force arrived on Planet Namek and Krillin sensed their presences. He thought it was Goku, but Nail said, "This is an evil presence that approaches." The Namekian Ki Sense was first displayed when Raditz approached Piccolo, who reasoned that the incoming power could not be Goku because "It's too horrible."
The Instant Transmission technique requires the user to home in on a ki signature before teleporting them to that ki signature's location or somewhere nearby. Meaning the ability to sense ki was a requirement (the Instantaneous Movement technique, however, does not require a ki signature or the ability to sense ki).
- Goku first learns the technique after drinking the Ultra Divine Water, but is only able to tell where Piccolo is; after training under Mr. Popo, he is able to master the skill to the point where he is not worried about Tien fighting Mercenary Tao because he knows Tien will beat him.
- Mr. Popo helps Goku, Krillin, Tenshinhan, Yamucha, and Chiaotzu master the Ki Sense technique.
- Roshi is shown to possess this ability at various points in the series.
- Master Mutaito may have possessed this ability given his knowledge of Ki-based techniques.
- At the beginning of Dragonball Z, Krillin had also learned the ability through unknown means, and by the time Vegeta and Nappa had invaded Earth, all of the Z Warriors had learned the skill. Vegeta had also managed to learn this ability after his battle on Earth.
- Dr. Wheelo is shown to be able to detect Ki. It is unknown if this ability is natural or mechanical, or a combination of both.
- Though Frieza and Cold are unable to sense battle powers without a scouter, Cooler is able (as it is required to use the Instant Transmission technique) to sense battle powers as well as hide his own immense battle power (allowing him to be able to sneak up on opponents like Piccolo). The Meta-Cooler clones appear to be able to detect battle powers likely through technological means.
- Android 19 and Android 20/Dr. Gero are able to detect and analize an opponent's battle powers much like a scouter. Given Dr. Gero has an organic brain, it is possible that he also possess a limited natural Ki Sense.
- Android 13, Android 14, and Android 15 all possess Ki Sense devices linked to Doctor Gero's Super Computer.
- Android 17 and Android 18 could not sense battle powers, although Android 16 had software in his design that worked much like a scouter.
- Android 18 may have later learned how to sense Ki from her husband, Krillin.