Instructions
Turn on your PS3 system. You can do this by pressing the power button on the front right side of the system or pressing the "PS" button in the center of the Sixaxis controller.
Wait for the PS3 system to boot up and reach the main menu. Once the menu has launched and you can navigate with the controller, the system is ready.
Press the "PS" button on the controller.
Look to the top right side of the screen and identify the controller's battery icon. A picture of a battery with a filled-in interior indicates a full battery. An empty battery icon indicates a low battery. Partially filled interior indicates the current battery level.