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Fuel injected cars actually don't have a fuel flow sensor. The fuel flow is computed from the time the fuel injectors stay open so you could tap into one of the fuel injector wires and figure out how long it's open as a percentage. Then multiply that figure by the injector's maximum flow rate (when 100% open). A mechanic can tell you that value as it's a known value used to compare against when injectors are taken out of the car an tested. Finally you multiply that by the number of injectors/cilinders in the car and you'll have a reasonably accurate measurement of fuel flow.
To improve on that you'd need to compensate for the injector's open and close time (deduct the time it takes to open and add the time it takes to close), which is something you'll probably have to figure out using one of those fuel injector testers.