There are U shaped steel pins that need to be retracted before the connector can be removed. Once off the injectors can be ohmed by putting the Multimeter in resistance setting for small ohms and set the probes onf the meter one on each contact tab in the injector. Meter needs to read between 10 to 14 ohms. The reading will read lower ohms when the injectors are cold.
Inspect each connector that you unplugged and look for green and/or white deposits. If there are then they need to be cleaned with contact cleaner and perhaps some burshing (no metal brush tooth brush ok to use). Then add some dielectric grease to the connector side (female connector slits) place the U sleel clip completly back on the connector (it is tricky to get completely on) and then press the connector back on the injector. 1 down 5 more to go.