Prepaid doesn't have detailed use billing. You'd have to keep a watch on your recents tab in your dialer app. That's where your call history is unless you delete calls made or received or factory reset .
New prepaid accounts (as of Oct 7 at least) are using a different system which does show call/text logs online.
I asked support about transferring my old account the new system and they said for now I'd have to port the number to a new sim and back. I think I'll wait till they fully roll this out, but it's great to see the change.