Rockhound
I will not speculate on what took place on the flight in question, but will give you some background on standard practice.
All transatlantic flights report their position on a regular basis on the crossing, and usually included in the position report is "fuel remaining", so that fuel is constantly monitored and reported throughout the whole flight.
Fuel is monitored both by the crew, and on modern aircraft, by the FMS as well.
As part of the checking process, the accumulated burn is compared with tank fuel indication and the flight plan burn, which should show up either excess burn or leaks depending on where the discrepancy lies.
Furthermore prior to departure, if the flight plan is loaded into the FMS and the fuel on board is not adequate, the FMS will alert immediately.
I am aware of one incident several years ago in my own airline, where a fuel line cracked and started a massive leak shortly after the aircraft had passed the Azores westbound. The crew picked it up when the tank indication was way down on the totaliser fuel burn.
They shut down the appropriate engine and returned to SMA, luckily they were only 1 hours flying away and were not that short of fuel.
edited for a surfeit of "ands"
[ 28 August 2001: Message edited by: SunSeaSandfly ]