Whether or not a cell phone is being tracked by an outside source is detectable by noticing if the phone is behaving oddly or if the battery runs down a lot faster than normal. Another way to tell is if there are unusual noises in the background of phone calls.
If a phone suddenly lights up when not in use or shuts down by itself on a regular basis, there is likely spyware running in the background. If the spyware is not running on the phone, there may be software accessing it through a Wi-Fi connection. Either way, these types of software drain batteries rather quickly or use more of the phone's data than normal.
Static, clicking noises, regular beeping or voices in the background that occur during most or all calls are indications that someone or something is tapping into the phone calls. Call listening and recording features interrupt the call signal because they act like a conference call leaving an open channel that creates noise. Also, if the phone is receiving unusual text message containing random numbers, symbols or characters, a remote control feature on the spyware is likely malfunctioning and sending coded transmissions to the phone that are supposed to be invisible.