You can tell it is charging when the single light on the top blinks. A single blink means that your WakaWaka Light / Power+ is charging slowly (but surely), two consecutive blinks means it is charging faster already and three means it is charging optimally. If it blinks 8 times in a row, it means that it is fully charged.
You can place the solar panel towards the sun or hold it under a strong lamp to check if the light is blinking and functioning properly.