Just bring Ki from your dan-tien, and touch your opponents, or whoever you want to sense power level of.  You can also do an exersize that helps you continually sense power level. Here it is: Just bring Ki for about 5 Miles around you, and continually keep it there.

 

Ki Object Scanning

Just fill the room you think it is in with ki, and visulize exactly what it looks like.  After a few minuets you should get a background of where the object is.

 

Ki People Scanning

Do the same thing as the Object Scanning but instead of an object, Visulize a person.