Mental ki-detection is when you can feel another person's powerlevel inside you. Basically, you can tell with your mind who is strong and who is not.This is the best method of ki-detection becuase it is more accurate. This has been done a lot of times in Dragon Ball Z. Go to our movieclips section to see examples. Anyway, anybody who is warrior is allowed to do this. Even Master Roshi has the ability. A matter a fact, he senses it more than anyone. So, here are the facts, once a fighter has a powerlevel and ki, they are fully able to detect Ki.
The second method is using a tool called a scouter.This can measure a powerlevel to it's exact number.The scouter is not always accurate. If the scouter is exposed to a really high powerlevel, it will overheat and explode. This is a insufficient way to detect ki. But people who can't detect ki, need it.