You don't need a halo item to sense Ki, although you're half write. Ki Sense is when you press Z without a scouter. The size of the spheres on your screen when you've pressed Z represent how strong the Ki of that user is, you can also find people with it if you know their specific Ki color, although I have no idea what the coloring means. When you see a big sphere that means that there is a very strong user, usually a Super Saiyan 3/God/Blue, Frieza final form, namek god or a Mystic Human.