As an undergrad, I used to make lists of errata for texts I have read and post them on my website (I still keep updating the list when I read new stuff with a nontrivial amount of errors). Back then I wasn't thinking of it strategically (it just felt like the obvious thing to do, like reporting bugs in software), but now that I've had some bits of experience with assessing students' applications, I have come to the conclusion that such lists of errata are fairly good evidence of actually having read and understood the books/papers you claim to have read. (I have seen my share of high-schoolers claiming to have read graduate textbooks, but somehow failing to remember much of use from them afterwards. It appears to be an inflationary currency, where everyone keeps claiming more and more achievements, just as with most other extracurriculars in student applications.)
Another thing that can help is posting collections of solutions to textbook exercises. But this is somewhat more hit-and-miss; the copyright status of such collections is unclear (they are derivative works most of the time), and some professors (older ones, in particular) might find it unwelcome (as they want solutions to not be easily available to students). A variant of this that lacks these drawbacks is posting little notes about the text, such as "a different proof of Proposition 2.4.1" or "why the second assumption in Theorem 2.5.2 is needed".
These days, you can also ask questions about the textbooks on forums like math.stackexchange, although of course these better be good questions.