The voltage at the pump will likely be pretty close to the battery voltage if you simply measure it whilst the car is stationary and the engine running. this is because the current requirments of the pump are much lower than when at full boost. an 044 pump for example takes around 17 amp at full load (i.e. when on full boost) and its at this time that resistance due to poor earths / slack connections / thin wire etc all have an impact on the actual voltage the pump sees.

as the 044 example pump takes over 200 watts at full load you want to try and simulate this condition so something like a 130 watt headlamp would do and this will simulate the situation that would occur whilst you are driving. in order to test the wiring (as opposed to other components such as battery and alternator) its better to measure the voltage DROP between battery and fuel pump rather than the absolute voltage that the pump sees. do this by loading the pump with a headlamp as described above then stick a digital voltmeter from the battery positive to pump positive. do a second test from pump negative and the car's chassis to test the earthing of the pump.

a second consideration is that many voltmeters designed for car use have a damped movement which means there will be a delay in response which may confuse the perception of the results. another problem is that many voltmeters designed for cars are miles out.