IPhones typically display the battery icon in the upper-right corner of the display. However, by default, they only display the battery state visually via a black bar within the battery icon. To get the percentage displayed next to the icon, a user has to navigate the settings menu.
An iPhone can display the battery percentage as long as it runs on iOS 4 or newer. To enable that option, tap on the Settings option. After that, tap on General (only if you are using iOS 6 and higher), and then select Usage. There, slide the Battery Percentage option to green (in iOS 7) or On (in iOS versions 4 through 6).