As to the last part, if people are sufficiently determined, you're in for a hard time, anyway. A 30-second screening at the border is pretty good at sorting out the Epsilon minus semi-morons, though. (Whose numbers, one would suspect, vastly outdoes the really determined ones.)

As for false negatives, I wouldn't be too concerned as long as you got to talk to someone reasonably well versed in your field. Also, as suggested by golergka and rconti - they probably use the questions as a way to elicit a response from you and judge that, more than your actual answers. That makes a lot of sense to me, now that I pause to think of it for a second.