I'm currently trying to teach myself math through reading. I've made a schedule to keep my pace and to make sure I finish enough problems. Should I add to my routine a timed test now and then?
I'm in two minds about this.
You should definitely seek to have your understanding challenged as often as possible. It's very easy to fool yourself into thinking you "get" something, especially as things become more complicated. Hence a degree of self-skepticism is a virtue.
However, there's a problem, and that's in marking your own answers. See my answer to this question: What's the best way to measure mathematical ability? Often in Mathematics, when you're right, you know you're right, but we're all human and this phenomenon happens less & less as, again, things become more complicated.
Marking is still subjective. It's not as mechanical as you might think. Mark schemes are only guides for the guesswork markers do.
What might seem "obvious" to you now might not seem obvious to others or even to yourself later on so, yeah, there's that too.
So what should you do? I don't know. I'm tempted to say you should learn to deal with doubting yourself. Being unsure of your solutions will help you develop the ability to spot your own mistakes, which is an incredibly useful skill. However, you might not be satisfied with that; indeed, you probably shouldn't be just yet; so let's say you should test yourself but have your solutions verified by someone competent.
Use Math SE :)
Definately. Even being taught by a teacher, I find that the finer points of a topic elude me until I do past papers or class tests