When I start working with someone early in their junior, the first thing I try to get figured out is whether they’re going to take the SAT or the ACT. I’d rather have them go through a couple of weeks of indecision early on than suddenly decide to switch tests after six months of preparation (especially because every year I do get people who’ve already been prepping for one test for six months, then decide to switch two weeks before the other and want to cram. That’s really not fun for me.) Usually I just tell my students that if they can’t stomach the thought of taking both a full SAT and a full ACT, they should just do a couple of sections from each test and see which one they like better.

I’ve realized recently, however, that at the extreme end, there can be a simpler litmus test, at least on the verbal side of things, and that test involves sentence completions. Interestingly enough, though, it has very little to do with vocabulary per se. The giveaway is how easily you can either 1) plug in your own words, or 2) correctly determine whether the word that goes in a given blank should be positive or negative — regardless of how many of the words in the answer choices you actually know. If you try a handful of sections and are consistently stumbling over this exercise by the third question or so, that’s a pretty good sign that you should seriously consider the ACT.
Here why: while having a good vocabulary will help you on the SAT, the sentence completion section isn’t just a vocabulary test. It actually functions as a microcosm of the Critical Reading section as a whole in that it also tests your ability to perceive relationships between ideas. Vocabulary can be memorized, but if you have difficulty sorting out the basic connections between ideas in a sentence or identify key pieces of information, the unfortunate reality is that you’re probably not going to develop that skill in a couple of months. If you can’t even figure out what sorts of words go into the blanks on relatively straightforward questions, how are you going to be able to consistently determine nuances between words on vocabulary-in-context questions or  nail the relationship between the authors’ ideas on Passage 1/Passage 2? I’m not trying to be harsh, just realistic.
I’m also not suggesting that this is a fool-proof method, just that it can provide some quick insight into some of the struggles certain students might have down the line. To be fair, it’s not that these skills are not tested at all on the ACT — they are, but they feature less prominently and tend to be tested in a somewhat less circuitous way. There’s no sense in making yourself crazy if there’s a less headache-inducing option available.