How would you prove to a skeptic that you can count to a million (and pronounce each of the numbers)? Proving you can count to 10 is easy since you can just rattle it off in a few seconds. This is the same sort of daily dilemma a tester faces: how do you prove something when you can't just churn every possible combo. Ultimately, you need to make some assumptions, and so you can view the real problem as "what assumptions are you willing to make".
A few approaches I've seen so far: