Limitations on application availability and system modifications have caused developers and hackers to find ways to modify the iPhone operating system (iOS) in order to allow full control over iOS. This process of opening iOS to full customization is called "jailbreaking." Once an iPhone is jailbroken, a user can install applications from third-party repositories and modify core functionality of the iPhone. Jailbreaking comes with risks, however, and Apple can deny services or void the warranty if it is found that your iPhone is jailbroken. If you are buying a used iPhone, you can check if the phone is jailbroken with a few simple steps.
Check for common apps that are installed after jailbreaking a phone. In particular, "Installer," "Cydia" and "Icy" are apps that are installed automatically when jailbreaking a phone using the most popular methods. If these apps are present, then the iPhone is jailbroken.
Check to ensure that the default iPhone apps are installed. These apps can be uninstalled on jailbroken devices so if any are missing then the iPhone is jailbroken. The default apps include App Store, Calculator, Calendar, Clock, Camera, Contacts, iPod, iTunes, Mail, Maps, Messages, Notes, Phone, Photos, Safari, Settings, Stocks, Voice Memos, Weather, YouTube. iPhones running iOS 4 or newer also include AirPlay, Game Center, Nike + iPod and Print Center.
Check that you are connected to the AT&T 3G Network. Jailbreaking an iPhone is commonly done in order to utilize other mobile networks. If it does not say AT&T in the top left of the iPhone screen, then the iPhone is jailbroken.