To test a smoke detector:

Press the battery-test button on the unit to make sure the battery is properly connected. If the unit has a battery that's more than a year old, replace the battery.

Light a candle and hold it approximately 6 inches below the detector so that heated air will rise into the unit. If the alarm doesn't sound within 20 seconds, blow out the candle and let the smoke rise into the unit. If the alarm still doesn't sound, open the unit up and make sure it is clean and that all electrical connections are solid. If, again, the alarm doesn't sound, replace the smoke detector.

To test a smoke detector battery:

Remove the smoke detector cover, typically by carefully pulling down on the case's perimeter or by twisting the case counterclockwise.

Locate and test the battery voltage with a multimeter. The battery can be tested in-place or removed from the smoke detector. As needed, replace it with a new battery.