One means (used to mean?) actually checking the LLM's output one means keep trying until it outputs does what you want.
That's not the case at all though. The LLM doesn't have a mental model of what the expected final result is, so how could it possibly verify that?
It has a description in text format of what the engineer thinks he wants. The text format is inherently limited and lossy and the engineer is unlikely to be perfect at expressing his expectations in any case.