20 amp breaker connected to 14 GA
Putting this trick first but it is my last resort.

To find wiring when I don't have my scanner, a simple stud finder with a voltage alert. Turn all the other circuits off then using the stud finder watch for the voltage alert to light up.  You can get within a couple of inches of the wire path through the walls and figure out where it goes. I have used this method several times where I was not planning on doing any electrical work.  It is cheap and works well but you do need to turn the other breakers off for this method to work well.

To answer your question, here are my normal methods to trace wire and find circuit breakers.

I use a Greenlee CS-8000 it injects a signal on the wire and can trace wires through walls and even conduit. This tool is expensive ($995) and not worth it for homeowners. 

When I am doing telecom I use a less expensive toner/tracer again made by Greenlee but there are lots of low voltage toner/tracers both of the above can be used with no power on the circuit or live. (The wand style for telecom is limited to phone line voltages not 120 volts.) These need to be in close proximity to the wire to use on 120 v wiring &amp; the breaker needs to be turned off.  These are less than $100.

The last circuit tracer I use is the cheapest made to find circuit breakers most of these cost ~60$ and require a live circuit to work you plug into an outlet with the transmitter and use the receiver to find the correct breaker.