With a multimeter you measure the resistance of each injector. Unplug the pto side injector connector while pressing in the locking clip. Then probe the two terminals of the injector. It should measure around 14.5 ohm.
The two high speed injectors in the middle are the tough ones to unlock the connector. You could also take the measurements from the large 8 pin injector connector.
Adobe Page 663 in the 2005 Shop Manual.
This is while the sled is idling?