How to Detect a Wireless Home Alarm System

Check the location of sensors and receivers. Depending up the exact mechanics, a wireless alarm needs to be in a specific location in order for it to function. For example, a wireless alarm that depends on infrared signals will be placed near a door or a window, where an intruder might enter. The shift in heat signature from the intruder's body will set it off. Or, if the wireless alarm is based on a motion-detection system, it will be placed high in the corner of a room so that it can scan the maximum amount of space. (Motion detectors usually send off ultrasonic signals, which return to them in a specific pattern. If that pattern is disrupted by someone entering the area, they will be set off.)

Look for a central receiver or main control unit. All the sensors in the alarm must send signals back to a central receiver, either via ultrasonic signals or infrared. The receiver then sets off the siren and calls the security company for help. Signals from the sensor have a limited range--usually 100 feet or so--which means they need to remain comparatively close to the receiver to work.

Examine the components for batteries or separate power sources. Because they're not wired to the home's electrical system, every component needs a battery--often a lithium battery--which will need to be checked and replaced regularly. The good news is that the alarm system will continue to function if the power goes out of the house. But it also makes spotting them easier, since the battery pack can't be hidden in the walls the way wiring can.

Check for an auto-dialer or similar device. Once a wireless alarm gets triggered, it usually has a system in place to automatically call the police or security company. (Some auto-dialers will also transmit a signal to the owner's cell phone, which works even if the phone lines are cut. Because they often need to be connected to a phone line, you may be able to spot them by checking any phone jacks in the home.