If you want to see whether you understand the SAT…
Try explaining it to someone else. Friend, sibling, parent…anyone. It doesn’t even matter whether or not that person is actually going to take the test. The only thing that counts is that they’re willing to humor you and sit with a Blue Book for an hour or two.
There’s a reason I always ask my students to explain to me why they’re doing what they’re doing: in about two seconds, it usually becomes exceedingly obvious whether or not they really get it. If someone says that they know but can’t really explain it, chances are they actually don’t. (For a great explanation of that phenomenon, see this article by Daniel Willingham). It’s one thing to shrug and say, “Yeah, that makes sense” when someone explains the answer to you; it’s something very different to work out all of the steps necessary on your own and explain them to someone else.
If you’re already scoring very well (high 600s+), I would argue that this is actually one of the most productive ways to study. Having to explain something to another person forces you to clarify your own thought process. Things you formerly took for granted suddenly seem bizarrely murky, and you start to wonder just how you know to do x instead of y. You have no choice but to break the process into smaller, more precise steps in order to explain why the answer must be B rather than E. The result is that you learn exactly what you do and don’t know. Furthermore, you gain an awareness of your own thought processes — an awareness that leads to a much stronger sense of confidence and control when you actually take the test.