I just got a free 50$ month credit developer+ account from Rackspace. However, I don't see that credit amount written anywhere on the Control Panel. How do I ensure that I've actually got the credit?
You are correct, there doesn't seem to be an easy way to tell what credits your account may have. We're working hard to promote developer.rackspace.com. I'll work internally to add the Dev+ credit in your control panel.
If you did sign up for Dev+ you definitely have the monthly credits.
EDIT: After digging around internally -- this is a known issue that is being worked on. You will see your credit reflected in your first invoice (and yes, if <= $50/month it's free!)