You can check your usage by logging into your account, and navigating to your Hosting page. You will see a summary at the top of the page.

Here you can view activity over the last month in terms of both storage and transfer.
Below this you'll see an aggregated (by month) summary of all charges, as well as a record of all "events" that have occurred against your account. By "event" we mean things like being out of credit, or sending you a notification to that effect, or automatically topping up your account - and so on.
