Certainly, you have reading comprehension passages on the SAT, GRE, and LSAT. But beyond that point, is it really possible to measure reading comprehension through multiple-choice tests? There are multiple motivations for why an author may make argument X, and multiple interpretations of his exact argument, which may be affected by the immediate circumstances that made him write that argument at that particular point in time (and while we may know a bit about the society that he lived in, chances are that we'll probably never know who he intended the book to be written for - hell - some people probably intended to write a book to impress some potential lover [and hell, maybe some passages in the book are motivated to attract the sympathy of that lover] even though they pretended to write it for the greater good of society).
I mean, all you have to do is to look at courtrooms. There, people pretty much argue about what people (in a case with precedent) actually meant, and there's usually no concrete (and correct) answer. And people who study the life of a particular philosopher may have multiple interpretations of why philosopher X wrote passage Y, and what X really meant in writing passage Y.
Plus, it's oftentimes very interesting to read the writing of someone you know REALLY well because you know their life so well that you can dream up hypotheses of what prompted them to write passage X at time Y (and you also know his social network so you could imagine what audience he had in mind, and what audience he might have secretly hoped to impress without mentioning it). And perhaps because of this, reading comprehension is not a skill with a single scale, since people are better at understanding some people than they are at understanding others.
I mean, I guess the ultimate test of reading comprehension would be to test someone on one of Immanuel Kant's books, or on some recent postmodernist book (made artificially difficult to read/parse), and then ask him "well, what is Kant trying to get at"? But is that really a valid test? Or maybe one where you have to dissect the entire structure of a book where X is pretending to be Y pretending to be Y pretending to be X (of course, such books are rarely written, but theoretically possible).
Alternatively, wouldn't someone with exceptional reading comprehension be better at reading crappy writing? (because they're probably more able to process multiple thought-streams at once - which includes possible fragmentary/jumbled ideas?) A common assumption in reading comprehension tests is that the writing is "perfect", but this is frequently not the case, and people (especially older people) can get attention lapses when writing