You will need
Check the temperature of the battery. If the battery temperature is high, then it is discharged. If this happens during a conversation, it is not necessary to worry. But if the battery stays hot in standby mode when no one touches about two hours, then inside of it is probably spyware.
Watch for a time sufficient battery power. If you begin to notice that you battery lasts very long, provided that you rarely use them, it may mean that there are outside processes. Possibly spyware.
Pay attention to the process of turning off the phone. If it takes suspiciously long time if while flashing the backlight, or after it burns off, or the phone did not respond, something is happening.
Note the behavior of the phone. If he can perform certain functions switched on or off, then most likely it's "the wire." Although it may be failures in the operating system, but not ruled out the first option.
Make sure if you can hear interference during conversation. If you regularly every time you hear the noise, petricciani, it is possible that your phone is tapped. In any case, show it to the specialist.
To determine that the phone is the tapping of an inexperienced user, of course, difficult. So if you really have concerns, you should consult a specialist.
Remember that over time the efficiency of batteries decreases and the reduction of working time is a natural process. It is worth reflecting that if only he recently worked on "perfectly", and now the battery barely enough.