There are all sorts of injector testers on the market to buy and there all very simple in how they work. If i were you id copy there design and it shouldn't take to much time or money.
Playing with injectors and common rail fittings can give parts of your body a diesel fluid injection. That will lead to local trauma and often a Clostridium infection.
Perhaps measure the exhaust temperatures. If they are all similar then injector volume is OK.
The alternative is to catch the spent fuel while counting 1000 injections and measure the volume.
All else being equal, shouldn't you be able to measure the explosive energy (by the displacement behavior of the piston) of the fuel air mixture and using molar fractions calculate the amount of moles of combustible released into the chamber?