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1) A 3-way switching arrangement will have 2 brass screws and 1 black screw on each 3-way switch. At one of the 3-way switches, the black screw will be the circuit feed from the electrical panel. At the 2nd 3-way switch, the black screw will be the "switched loop" conductor that will go to the ceiling light. If the conductors are installed in conduit, you can gently tug on one of the 3-way switches black screw and listen if the wire is connected to the ceiling box. Turn the breaker to OFF prior to backing the switch out from the wall.
If the house is wired using Romex cable, then you would need to turn the circuit breaker to the OFF position and use a metal stud finder to locate the ceiling box (assuming the ceiling box is metal and not PVC plastic) or use an audible toner tracer. See answer below on 3A)
If the house is wired in Romex and the ceiling box is PVC plastic, you can also use a toner/probe to trace the wires. See my response below on answer 3A.
1A) 3-way switches can also be used to control a switched wall receptacle. If unable to determine if controlled by a ceiling light, check your wall receptacles with a 3-prong cube receptacle tester to determine if they are constantly hot or possibly switched.
2) If the ceiling box was buried in drywall or some other covering, that is a violation of the National Electrical Code. All wall, ceiling, splice and pull boxes must remain as "accessible" per code.
3) For the dead light socket, you can tug on the wires if the house was wired in conduit. Turn the breaker to the OFF position prior to removing the light socket.
3A) If wired in Romex, you will need an audible toner and tracer probe to trace out the controlling wall switch. If you know which breaker controls the light, turn it OFF. You will need a toner such as the type shown on the link below which is available at your local Home Depot. Lowe's also sells a similar tracer model:
3B) With the circuit breaker in the OFF position, you should also have a 2-prong voltage tester or an AC voltmeter to check the voltage at the light to make sure the light is not wired using a shared neutral from another circuit. If no shared neutral, place the alligator clips onto the hot and ground wire at the light. Then go around the room or nearby rooms with the probe and listen for the audible tone. If the wire is connected to the switch from the light, you will locate the wall switch.
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