In Australia, hardwired smoke alarms, also known as mains or 240V alarms, are usually indicated by a small green light on the device, indicating that it is connected to the building's power supply. In addition, hardwired alarms usually have a backup battery (either a 10-year lithium battery or a replaceable 9-volt battery) to ensure continued operation during power outages. If you are unsure, a licensed electrician can verify the type of alarm and its correct installation.
Here is a more detailed explanation put together for you by anke security:
Green light:
The green light on a smoke alarm is a common indicator that it is hardwired to the main power supply.
Backup battery:
Hardwired alarms usually have a battery as a backup, either a long-life lithium battery or a replaceable 9-volt battery.
Wiring:
If you can see a wire connecting the alarm to your home's electrical system, it is a hardwired alarm. However, never touch the exposed wires without turning off the power at the switchboard.
Age of property:
Homes built after 1997 or extensively renovated are more likely to have hardwired alarms installed, as required by the National Building Code.
Testing:
A hardwired alarm should continue to operate when the battery is removed (and the power is on), or it may sound a low-battery warning if the backup battery is low.
Professional Inspection:
If you're unsure, a licensed electrician can inspect your smoke alarms to determine if they are hardwired and properly installed.