People can regularly check circuit breakers before they fail or check breakers that have tripped with a digital multimeter. They must first unplug all appliances leading to circuit breakers they are testing.
After appliances have been unplugged, people testing circuit breakers remove the cover on the breaker box with a screwdriver to access the power lead to the breakers. They then set the breaker being tested to the "on" position, and set the multimeter to read voltage. The red power lead on the multimeter is touched to the screw that feeds power to the breaker, and the black probe on the multimeter is touched to the grounded box. The multimeter should read 120 or 240 volts if the circuit breaker is working properly.