The same way we've always done it - glance at it and see if the numbers look like they're within an order of magnitude of what looks reasonable.
so what if there were some numbers in the report which are in actuality, an order of magnitude or two outside of what you think is reasonable, because something was wrong, but the AI agent reports something that looks normal?
So as long as the LLM only makes errors in the single-digit percentage range, everything is peachy. Make number go up, but not by too much.