A lot of mobile devices give you a notification message when they are completely charged. This isn’t the case with the iPhone, iPod touch, and iPad. Here’s how to tell whether the device is fully charged or not.
This is pretty pointless if you have an iPhone 3GS or iPhone 4 because there is a battery percentage meter, but on older devices, it’s a handy tip. The easiest way to tell is by taking a look at your battery meter in the top-right hand corner of the status bar.
If there is a lightening bolt inside the battery, that means the device is still charging.
When the device is finished charging, there will be electric prongs inside the battery. This means that the battery is 100% charged.
For clarification, take a look at the screenshots below:

