Fuel flow baseline
One simple check is to compare eng #1 vs #2 (vs #3 vs #4) at matched EPR (or N1 as the case may be). Keep track of the FF difference from flight to flight - this will require setting up an informal system for sharing data between crews.
Note that the thirstier engine never will decrease FF - if the delta value decreases it will ALWAYS be the "good" engine getting thirstier, "catching up" to its mate.
Also - many years ago the engine makers used to produce slide rule (maybe circular slide rule) computers to represent a nominal engine on wing. I developed one that fit in the F/E's shirt pocket.
They worked pretty well as a crude trend monitoring tool.
But personally, I'd insist that the dunce who pulled the tables ride along on every flight so he understood what he hath wrought.