One interesting idea: the College Board could add in a class of problems requiring a particular novel, non-obvious approach that are effectively poison pills. Students who take the SAT prep courses are drilled on how to answer them, while students who take the test naively are bound to answer them incorrectly. It doesn't affect the actual score reported to the student. But when the score is reported to universities, a shadow score that represents likelihood that the student received extensive prep is also reported, which gives that kind of context.