With registrations, it is easy as you do get reminders about renewals (even if set to auto-renews). It's not so easy with "hosting" for a given domain when many domains are hosted on the same account; the "anniversary" that drives billing is the one for the "master" account - and with unlimited extra domains, it is easy to get confused.
With a little planning for the future, you could make yourself an easy to track system by just adding a text file to the user space with the creation date of each domain, and update it when you add a domain.
Or just "touch" a "created_on_date" file when you create the domain, and leave it alone - it's file date from ls will reveal when it was created.
I know, those are "after the fact" suggestions - I don't really know offhand of a way to check from the panel or the shell.
Have though of going through your email (or your "Support History" in the panel) to find the "Domain whatever.tld has been successfully created" message that is sent upon domain creation? That ought to work, in a pinch.