When you declare a fuel emergency or even urgency, there's often follow up to figure out why (mechanical issue? problem with dispatch? problem with flying technique? exceptional weather condition that could be forecast better? etc). And there is plenty of data in aviation to know what happened.

Dispatch knows how much fuel they say they put in.

Your flight time, speeds, and profile are known.

ACARS may be reporting fuel use throughout the flight.

etc, etc, etc.