Test it! Push the test button on the front of the unit and it should emit a loud beeping sound for as long as the button is depressed.
If you have a smoke alarm with batteries:
Smoke Alarms powered by long-lasting batteries are designed to replace the entire unit according to manufacturer’s instructions.
In standard type battery powered smoke alarms, the batteries need to be replaced at least once per year and the whole unit should be replaced every 8-10 years.
In hard-wired, battery back up smoke alarms, the batteries need to be checked monthly, and replaced at least once per year. The entire unit should be replaced every 8-10 years.