Are you talking about locating a cable that is behind a finished wall, sort of the way one would use a "stud finder" or determining what receptacle/light/etc. goes to which breaker, as in "who put this thing in and never labled the service panel?"
I search to find a good one and even posted a thread here asking for feedback. You may want to search back a bit to find the thread. I ended up with an Amprobe Pasar AT- 2004. This retails for about $900, I paid $450 on Ebay, brand new. I used it the other day to trace a wire through a finished house and it paid for itself that time alone. I was looking at some Greenlee tracers that were a little less and may be worth a look.