There are quite a few different spy programs on the market that are designed to covertly track or monitor cell phones. Some of the most popular include MobileSpy, MSpy, and FlexiSPY.  Most of these programs do a fairly good job of hiding themselves, which can make it difficult to determine whether or not your phone is being tracked or monitored.
If you pay close attention to your phone's behavior, you may be able to spot signs that could indicate that it has spy software installed. Keep in mind, however, that many of these issues can also be attributed to normal cell phone behavior, so there is no guarantee that the software is installed on your phone even if you do notice these signs.
- Shorter Battery Life. Most spy programs run continuously on your phone, which can cause the battery to drain more quickly than usual. If you notice a sudden change in battery life, it may be because a hidden program was installed. Remember, however, that batteries naturally hold less of a charge as they age. A slight decline in battery life is normal. A sudden change in battery life, on the other hand, could be cause for concern.
- Strange Text Messages. Tracking software is often controlled remotely. To accomplish this, a remote controller sends a message to the phone that contains instructions.  Even though these messages aren't supposed to show up, they sometimes may find their way into your inbox. If you notice that you are getting messages that contain strange characters or random symbols, it could be due to a spy app on your phone.
- A Sudden Spike In Data Usage. If you notice a sudden increase in your monthly data usage, it could be because a spy program is using your data to send information about your phone to a remote location.
- Jailbreaking. iPhones have a built-in feature that prevents software from being installed that doesn't come directly from the App Store. In order to bypass these restrictions, someone would need to "jailbreak" your phone. You can try looking for apps such as Cydia, Icy, Installer, Installous, or SBSettings.  These apps are used to jailbreak phones. If you did not jailbreak your own phone and find one of these apps on it, it is usually a good sign that someone else was messing with your phone and possibly installing spy software.
- Strange Behavior. Along with watching for strange behavior from your phone, you should also watch for strange behavior from the people around you. If you notice that someone in your life doesn't act surprised when you tell them something that you just found out through your phone, or if they always seem to know what you've been up to even if you haven't told them, they may be watching your phone.
This video has lots of other great tips on what to watch for: