The scouters are devices where you can tell the percise power level of a enemy/good guy. There rarely seen from the Trunks saga on, if so then the guy is pretty weak. Heres a list of the characters that's had a scouter. I have no clue about the Dragon Ball era.
Nappa
Raditz
Vegeta
Kiwi
Bulma (fixed one up)
Dodaria
Zarbon
Frieza
Jeice
Recoom
Burter
Caption Ginyu
Goldo
King Kold
Cyber-Freiza (had one installed into eye)
Frieza's Troups