You can check your remaining credits by going inside any domain, then scrolling down to the Advanced features box:

The credits shown are cumulative for your whole account, not only for this particular domain.

You can also see usage graphs on each of our nameservers for the last ~ten days by clicking on “View statistics of your credits usage” in the same “Advanced features” box. The graphs look like this:

Each graphs shows the activity on one of our nameservers.