If you are considering buying a used iPhone and want to know whether or not it has been jailbroken, there are few ways you can find out. The quickest way is to see if there is a Cydia icon on the home screen. If so, then you know the iPhone has been jailbroken.
Another way to see if the iPhone is jailbroken is to look for unfamiliar apps on the home screen that aren’t available through the Apple AppStore because apps that aren’t available through Apple won’t download unless an iPhone has been jailbroken. Finally, you can tell if the iPhone is jailbroken if a pineapple or skull shows up when you start the iPhone instead of the Apple logo.
Source EvasionJailBreak