WARNING: check smoke alarms with smoke only, not by pressing the self test button.
The self test button which people press to test the smoke alarm is not a proper test because it does not test the SMOKE SENSOR. It only checks the batteries and the bleeping siren.
Therefore if there is a fire in the house and the smoke sensor is faulty on the alarm, it will not work. What happens then ???? You can get smoke matches ( as shown in picture ). You can even blow lots of cigarette smoke at alarm or any other means of smoke.
I hope this information is helpful and it could save your / families life.