The answer to 'how to test circuit breaker' is to use a circuit tester to check if the breaker is functioning properly. This involves turning off the circuit breaker, then using the tester to check for power on both the line and load sides of the breaker. If the tester shows no power on the load side, the breaker is likely faulty and needs to be replaced.
It's important to take safety precautions when testing circuit breakers, such as turning off the main power supply, wearing rubber-soled shoes, and avoiding contact with live electrical parts. Testing circuit breakers should be done at least once a year as part of regular home maintenance, or if you suspect a breaker is not working correctly.