The balance and expiry date of your gift card can be checked at any time on this website (at the bottom). You will find the power no. and the code on the back of the gift card. You do not need to enter the card number to check the balance.
The "Power-Nr." is the 8-digit number, which starts with 54.......
The "Code" is located in the bottom right corner. This code must be carefully scratched open.