I am using an inexpensive VAC/DC Sperry 80-500 circuit tester and a detector that tells me which circuit breaker in the panel controls the outlet. House (northeastern USA) was built in the 1940s.
An old wire with two conductors comes up from the basement and terminates at a two-prong outlet in the foyer; a wire with two conductors comes down from a light switch on the wall that controls the light in the foyer ceiling. That wire is also attached to the two-prong outlet.
I plug the line-tracer into the outlet, its red indicator light lights up, and then I go down to the basement. The device makes a rapid buzz at breaker #34 in the box. I flip the breaker OFF and come back upstairs to find that the red indicator light of the tracer plugged into the outlet is now OFF, as expected.
But the foyer ceiling lamp is still ON.
I detach the four wires from the outlet and the ceiling light goes OFF.
We now have these four bare wires down at the baseboard in the foyer:
from ceiling light switch xxxx xxxx xxxx | | C D A B | | xxxx xxxx xxxx from basement
With the circuit breaker in the basement still flipped to OFF, and now using the circuit tester:
A + B = no light on the tester
C + D = no light on the tester
A + C = faint light on the tester
A + D = bright light on the tester
B + C = faint light on the tester
B + D = faint light on the tester
Does this mean that there is electricity flowing through the wire that comes from the ceiling light switch?