Pull two injectors from the motor but still attached to the distributor. By-pass the fuel pump and press down on the airvalve. Compare the flow. Place it into a graduated cylinders for greater detail. Remember that if you do this for any length of time that all injectors will be pouring. Take them all out and catch all the fuel if you want to compare.
We use a device called a differential flow meter to make these measurments
Bosch Master, ASE Master, L1
33 years MB technician