FOB at the time of the check PLUS fuel burnt = roughly the amt in the tanks prior to departure that you logged (taking into consideration taxi fuel etc).
I think the above came about following the Azores gliding incident. Simple enough to do and our OPS manual has us doing it at least every 30 minutes and preferably over a waypoint to give a better guestimate. As an aside as part of my departure briefing when I am flying I will make reference to the expected fuel that we will be landing with plus what we need for the alternate. In flight I keep an eye on the expected landing fuel (bottom right on the MCDU). If it has dropped then I start asking myself questions (lower FL, stronger than forecast winds, vectoring by ATC, change of runway, or possibly a leak if none of the previous apply).
Other things I might log is a comparison of the forecast winds and ISA to actual. If I get time in the cruise then I would haul out the sig wx and winds aloft charts. Possibly consider asking ATC for ride reports as well although generally I will get beaten to it so I can just listen in on frequency. After all of that we can determine whether it is better to go right up to the optimum as per the FMGC or perhaps staying a bit lower for a smoother ride.
Now if your on a line check and really want to earn your brownie points then you can annotate the flight plan with the FIR boundaries from the high level charts so that you don't miss any handoffs and find yourself flying in close proximity to any F15's! Whilst you have your head in those you might as well look out for any restricted or danger areas, GRID MORA's etc as well. After all of that just remember to land, preferably in the right place at the right time
Enjoy.