The maximum fuel imbalance is usually calculated by the Flight Sciences group as a reasonable limit (from offset lateral CG) and set in the EICAS/ECAM as a Caution message. The flight test is to show that the AFCS can cope with that offset during critical maneuvers such as during approach - for example fuel offset is usually one of the limit conditions flown during the 30 approaches to demonstrate that the Autopilot meets CAT II requirements. Don't forget that most systems return unused fuel continuously back to both wings - the amount held in either one is determined by the relative input & output flows. A fuel leak is flagged by the difference in fuel used (flow into the engines from flow meters) relative to the total fuel used (from the Quantity & Guaging system's level sensors) and is as likely to be by the difference in rates as actual quantities.