You can always see your current balance by navigating to your wallet. You can find all transactions in the transactions overview including all "top-ups". In this overview you can also see previous transactions when renting a shared bike or buying a ticket.
In case your wallet balance is insufficient to cover the expenses made, your balance will turn negative. When this happens a direct debit will be implemented to restore your balance to €0,00. You can prevent this from happening by making sure your balance is sufficient, or by directly topping up your balance when it's negative.