How to know when my laptop is fully charged when it is powered off. My laptop is Lenovo G555?

If it is powered off and the PSU is plugged in and on then the internal battery control will stop at the optimum level for your battery and although there may be LED's showing on the PSU and your laptop there's very little happening until your battery power level drops to that where your internal power control lets the PSU recharge the battery, this is normally in the upper range of the battery so that it is kept at the best optimum level at all times.
when you have the PSU plugged in and on then use the laptop the battery is only used IF the PSU loses power or is unplugged but the PSU will continue to keep that battery at optimum charge level.

LED lights on your Lenovo laptop will change after it's fully charged