There is systems and software in place. Are you asking just to know, or are you asking so you can spy on someone. Tracking someone without their knowledge is against the law and is a crime that you can go to jail for. If you are honestly thinking about stalking someone, then don't. Would you want someone following you around, judging everything you do? There is not only a legal aspect to spying, but a moral one as well. If you feel the need to spy on someone, then you shouldn't be around that person to begin with.