There are two ways to check the charge of your battery, the first is by looking at the battery bar on your display, which will go down as the battery drains. The battery bar will only be shown when the display is turned on.
You can also check on your battery itself by pressing the button towards the bottom of your battery on the same side as the lock. The indicator LEDs will light up depending on the level of charge. There are four total lights, one red and three green, and if they all light up, you have a full charge. As the charge level gets lower, the green lights will disappear one at a time, and if you only get the red light, the battery is getting low and must be recharged. No lights at all indicates your battery is completely empty and will need to charge for a while before the indicator lights will work.
The LED indicator lights will only work when the battery is turned on.