"Ki-detection" started off not being very specific, but as the series went on, it became more fine-tuned, partly for the development of the story, and of course it is logical to expect its practioners to get better at the art.

It is somehow a given in oriental cultures that if someone is good at martial arts, he/she must possess a good ability of sensing where his/her enemies are. (When someone has the intent to kill, they give off certain kinds of 'energy').

First, when DBZ just started, Radditz, Nappa, Vegeta, and Freeza's henchmen used "scouters" to find out people's powerlevel and where its coming from. Scouters are small machines that you wear on your head and tells you ki information. But then as the series went on, people got stronger and can sense ki without scouters.

Near the end of the series, ki-detection is pretty well defined, one not only senses the level of power someone else possesses, they can only get a hint of that person's personality (i.e. good, evil, etc.), and by that personality, they can also get somewhat of a "signature" from that ki. We see evidence of this after Piccolo re-merges with Kami-sama. Cell powers up, so his ki can be sensed by the Z Warriors, and because Cell has the cells of the other fighters, their ki-signatures can be sensed from his ki. Later, Trunks notices the new Piccolo's ki, but doesn't recognize it (Trunks didn't know Piccolo very well, so he was going more by ki-level), while Krillin quickly recognizes it as Piccolo's ki. Another example is when Goku tried to teleport to Namek. He needed to look for ki-signatures that were similar to that of Piccolo's in order to teleport to the new Namek.