A circuit breaker finder is a line tracer used to trace electrical circuits, and used to be too expensive for most homeowners. Now for less than $40 you can have your own circuit breaker finder. Flickering lights article.
One model I use is an A.W. SPERRY CS-500A CIRCUIT BREAKER FINDER. It consists of two parts, a transmitter and a receiver. The transmitter uses power from the circuit you are checking and the receiver uses one 9-volt battery, not included.
To start, install the battery and plug the transmitter into the circuit you want to search. Turn the receiver on, just one click, the led will light and the unit will be in the maximum sensitivity setting. Place the receiver near the transmitter and the unit will start beeping. Now you are ready to go to the electrical panel and search for the controlling breaker or fuse.
Slowly move the receiver over the breakers or fuses while continually lowering the receiver’s sensitivity by rotating the wheel until it beeps when only over one breaker.
I recommend starting at the computer first so you do not turn off that circuit while the computer is on.
The transmitter has a light built in it so when you find the breaker and turn it off you can verify that you have found the correct circuit. The receiver will beep when placed next to receptacles on the same circuit as the transmitter.