On the A320 we use the FUEL USED indication. Since the FF transducer is located very close to the engine, if the F USED value is greater than on the other engine it would indicate a leak downstream of the transducer, hence in the engine. If the value is the same, it would indicate a leak before the transducer i.e. not in the engine.
I think you can access the F USED indication on the 737 by pressing something on the Engine Display if my memory serves me well. This was a B737-3/4/5 i.e. dunno about the NGs.
Another point re: fuel management is that a good check is that for every fuel check, add FOB + F USED and this should be equal to your departure fuel.