I'm mostly self-taught and while I know (and use) many advanced mathematical topics, I often enough find holes in my understanding of lower level math. Is there an exam (or series of exams) I could self administer to evaluate where I need to focus to improve my understanding?
The scope of such testing should run from elementary algebra and geometry through advanced linear algebra and partial differential equations. It isn't that I need to study the entire topics, just I think I've skipped or forgotten small concepts here and there.
I think what you are doing right now is fine. If you need something, read up on the matter. In my experience, when trying to learn "preemptively," it will often happen that when you need something you'll have forgotten (or perhaps there is now a better way, or new textbooks/lecture notes have shown up).
On the other hand, if there are areas that you repeatedly find have holes, get serious on studying them. The lecture notes by William Chen cover much of the undergraduate curriculum in clear, approachable style. I'm sure you'll find other suggestions digging here.