You memorize the answers? Like, I had an entire week of third grade where they wanted us to do a table of 100 basic arithmetic problems, and it was really a test of whether you remembered that 8+9=17 rather than some sort of actual understanding of what “eight”, “nine” and “the combination of eight and nine” meant.

I think arithmetic in general is actually one of the most difficult things to test understanding of, because there’s not a whole lot of understanding involved. It’s kind of axiomatic for the rest of math to build on top of. I would attribute “understanding” more to being able to synthesize useful results from properties like addition and subtraction being inverses of each other, being able to form relationships like “two small numbers added together are always less than two big numbers added together”, etc. But I did actually start this thread by asking how you would test for this, because I believe doing so is quite difficult. I don’t think I’ve seen many a test that people haven’t tried to game without “understanding” the subject.