The battery icon in the status bar will change from a lightning bolt to a plug icon.
Mine did not do this. It says at 100% and still shows a lightning bolt. Don't know if it's because I needed to leave it charging more or what.
The iPad shows a plug icon when fully charged.
Then the iDevice's battery isn't fully charged yet. This can happen if the battery calibration is off.
Unless you have a iphone 2g or 3g why wouldn't you show the percentage?
Like the other poster said when the big battery in the middle of the screen is full.