What is a visual indication that an iPhone is locked?

A passcode input screen serves as a clear visual indication that an iPhone is locked. When the device is locked, it requires the user to enter a passcode to access the home screen and apps. This screen appears when the iPhone has been locked manually by the user or automatically due to inactivity, emphasizing security by ensuring that only authorized users can gain access to the device's content and functionalities.

In contrast, a completely black screen does not necessarily mean the device is locked; it might simply be in sleep mode or turned off. Continuous vibrations could indicate notifications or alerts, but do not inform the user about the device's locked state. Lastly, while a red battery icon indicates that the battery is low, it has no direct correlation with whether the device is locked or unlocked, as it could appear in either state.