hi,
to find out your credit deal the 1223 or send a sms with NO text (or with YP, but i#m not sure if it works)
and to recharge it, buy scartchcards while you in greece or take this internet adress
https://eshop.myq.gr/eshop/ordertime.asp (but also not sure if it works with a german credit card)
I hope that it will works and I could help you!?