The ability to sense chi is something present in many fighter characters of Dragon Ball. It appears to be a skill one simply 'picks up', because the teaching of it is never shown and many characters just seem to be able to do it. The first instance it was used was towards the end of Tale 4 They Call Him...the Turtle Hermit!, when Son Goku says he can "feel" the presence of people in a supposedly uninhabited town.
The World Trade Orginization and those connected to it don't appear to be able to do this (except for perhaps Gurd) and instead use technology to detect what they call a 'Power Level', which is a numbered reading of a person's chi level. After Vegeta's battle on Earth he learned about their ability to sense and cast away his scouter permanently, so he could teach himself to sense.
It appears to be some form of Psychic Ability.