Are your smoke detectors hard wired or battery (only) type ?
Easy to tell – if the smoke detector has a light that is illuminated constantly, it is hard wired. Hard wired detectors are hooked to a 120 volt source constantly. Most have a battery back up in case of power outage. The battery backup is usually AA batteries, or a 9 volt battery, or may be a internal 10 year battery. Wires and plug are apparent under the detector when you take it off.
Battery type detectors, that either have a 9 volt battery, AA batteries, or an internal 10 year battery, do not have a constant light illuminated. Battery type detectors typically only blink with a red light once every 60 seconds. No wires under the detector.
Note: In the city of Los Angeles, any attached home is required to have all smoke detectors hard wired. How to check ? There should be an illuminated light on and visible on each detector. Other cities also may require hard wired smoke detectors. Carbon monoxide detectors do not have to be hard wired (though may be) in Southern California.