It's actually impossible to train gate agents to do that, as the occasional slip-ups demonstrate. A whole specialty in psychology, aptly named aviation psychology, exists to reduce these slip-ups. When a gate agent's attention wanes for a moment, the worst outcome may be an unintended trip to New Zealand. When it happens to a pilot, that outcome can be a crash with hundreds dead.

Given that there's a lot of excellent research available into design techniques that minimize these errors, airlines really ought to spend a little to improve their system.

(I know a bit about the field because my father worked in it in the fifties and sixties, long before it had been named.)