The circuits in a home or other domestic setting are quite complicated, and many advanced repairs must be performed by an electrician to ensure that your domestic electrical system remains safe. However, you can perform simple checks like testing an outlet or circuit breaker on your own. You can test any circuit, whether you have turned it off at the breaker and want to ensure that it is not actually live or just want to test it at the outlet to see if it's working.
Obtain a multimeter or a simple circuit tester; a multimeter is a versatile tool that can take multiple types of readings, while a circuit tester will simply have a light that becomes illuminated when a circuit is live.
Ensure the multimeter is set to check voltage if you are using this tool and not a basic circuit tester. Insert one probe of the multimeter or circuit tester into one side of an outlet on the circuit you want to test; place the other probe into the other side of the outlet. Hold the probes by their plastic, insulated parts to ensure that you don't receive an electric shock from a live circuit.
Check the screen or dial of the multimeter. If it shows a reading -- usually around 240 volts in a domestic circuit, the circuit is live. Note that if you are using a basic circuit tester, it will simply light up to indicate a live circuit.
If you plan to perform any work on a domestic circuit, test it to see if it is live, then turn off the circuit breaker that corresponds to that circuit and check it again to ensure it is no longer live.
If you find a circuit that should be working is not, it is best to call a professional electrician to fix the problem rather than attempt to fix the issue yourself if you don't have a working knowledge of domestic wiring.