Older models have a charge indicator on the top edge of the device. The indicator light is orange when the device has power and is in use. While charging, the indicator turns red; the indicator light turns off when the device is fully charged.
New models display charging information in the status bar at the top of the display. The indicator is shaped like a battery. When you use the device, the indicator shows between four and zero diagonal lines, with the number of lines decreasing as the battery runs down. During charging, the indicator shows a lightning bolt symbol. When the battery is fully charged, the indicator shows "100%" written on the battery icon.