Where can I check to see which kind of deposit type my account has? / If I don't use my funds, where do they go?

Under the "Payment" section of your "Account Settings", you will see that you can view your wallet details. Scroll to "Deposit Type". "Top Off" means that your funds will expire at the end of the month and not roll over to the next month's deposit. "Additive" means that your funds do not expire and will be added to your new deposit next month.