If you've recently noticed a severe drop in battery life, your phone may be in use when you aren't using it. Also check the temperature of the phone. You notice the phone get hot after extended use, see if it ever feels this way when you're not using it.
Monitor unusual activity by your phone. If your phone suddenly starts turning on and off by itself, has difficulty shutting down or won’t shut down at all, it might be bugged.
Step 3: Watch for coded text messages
Watch for coded text messages. If you get a text message that seems like data coding, with numbers and symbols instead of words, this could indicate misdirected attempts at data transmission by a third party.
Listen for background noise when you’re on a call. Echoes, static, or clicking sounds may be caused by line or transmission interference, or by equipment used by someone tapping your call.
Inspect your phone bill every month. If it indicates a spike in text or data transmission, someone else may be using it.
If someone has had access to your phone in the past, they could have installed a program named 'mobile spy'. If this is a possibility, reset your iphone to factory and you don't have to worry about this program.