How do you know what wire is doing what when in a very confined place? I had my electrician come over to help me figure out how to wire a receptacle. How did he know what circuit was what up there in this little area. Did he read the romex and find 12/2 and know that was the best circuit to run? I have been trying to trace down circuits for days and he just jumps up there and minutes later a wire is coming out of my receptacle box. I guess this is what 30 years experience gets you. He basically told me my house is an exact copy of 100's of other houses in the area.
Sometimes all it takes for a professional is to open the box and see whats inside to know everything he needs to know.
Chances are good he just extended a circuit.
If there are multiple circuits in one box and he magically knew which one to pick is a bit more laughable. He may have just guessed? He may have seen this exact box, in the same exact location, in a house exactly like yours, in your exact neighborhood. Laughable, unless you live in a tract house and every house on your block is identical. Is this true?
How does one actually know?
You should have asked him to explain what he did.
He always answers questions with stories, vaguely related to the question. Even if I had his hand in a vise he could never really answer the question. I asked him a simple question about outlets not needing to match the circuit. He went on for 25 minutes about aluminum wire. Then he went on about the codes for knob and tube. He is an old friend but you just can not get an answer out of him related to the question. He extended the outlet circuit because when he left I went through the breakers and it cuts power on a circuit that only feed three other outlets. He probably did just guess, I was not sure if he was hiding some sort of tester he had in his pocket.