It is highly likely that your smoke alarms were fitted by an electrical contractor or perhaps your new home came already installed with Brooks smoke alarms. In most cases, new homes or significantly renovated homes would be fitted with mains powered smoke alarms as a first choice.
The easiest and quickest way of making this determination is that the Brooks Mains powered smoke alarms will have a permanently illuminated green indicator when the mains power is connected. The 10-year Lithium battery models do not have this.
Other distinguishing features are that the mains powered smoke alarm “slides” onto its base which contains slots for the power pins on the alarm head to locate into. The base plate will also have a 230Vac warning label visible when the alarm head is removed. The 10-year Lithium battery models rotate approximately 5° to 10° and then drop out of their bases.
Lastly, the mains powered smoke alarms have a larger diameter than the battery powered alarms. Of course, unless you have one of each, you would not be able to make this determination with ease. The mains powered alarms are approximately 152mm in diameter whereas the 10-year Lithium models are only 115mm in diameter.