When dealing with electrical wiring of any kind, it's never safe to assume that the power is switched off. There are only two ways to ensure that electrical current is off to a set of wires: The first is to cut all means of electrical conductivity to the wires; the other is to use a circuit tester touched to the wires to indicate whether wires are hot or not. Circuit testers are available in small utility-tool-sized units or in larger box-shaped configurations. Homeowners are more likely to have the smaller circuit testers, while professional electricians often use the larger commercial-grade units.
Ensure that the power is either off or on, depending on which setting is needed at any given time. Acquire a hand-held circuit tester to determine whether there's current to the wires.
Hold the insulated handles of the two wire probes; touch the black probe to the grounding wire, then touch the end of the red probe to a bare section of the hot wire. Modern hot wires are usually red or black with white stripes. A positive indication of the wire being live is either a light activating on the circuit tester, a digital readout that indicates this or a needle moving to indicate power flow.
Note the location of the hot wire on a wiring diagram or mark the wire with a piece of tape. Test any other wires to ensure whether they have current to them or not; record them as well.
Insert the two probes into the two slots (for power receptacles); if the tester needle indicates there's power or the tester's light activates, then there's power to the receptacle.
Readouts with needles or digital readouts are better than light-activated testers, since they provide the amount of power reaching the tester (they don't just indicate that there's power).
Be careful when using a tester that there are no loose connections, frayed insulation on the handles or other damage that could cause injury.