Originally Posted by Nephoi

without going to much into detail, which I can't, but the fuel quantity is more or less constantly shown on the monitors in the cockpit. to read back the fuel quantity loaded (and compare that figure to the planned fuel loaded) is a part of at least one pre departure checklist. (water AFAIK) not.
IIRC also the MCDU warns you if the fuel loaded isn't enough for the route entered. but pilots may provide a deeper insight here...