If you have only partially paid the bill, or have paid too much, you can see this under "Bills and Usage". If you have overpaid, a credit is posted to your account and automatically applied to your next bill. If you have paid too little, the bill remains partially unpaid and payment must still be made. You can see the amount unpaid in the individual bill.
Tip: Use the convenient "Pay now" button in my Lebara under "Bills and Usage" to avoid making a payment that is too high or too low.