CIS injectors can be pulled and observed -- they operate based on fuel pressure. Digifant injectors are electrical -- basically they are a solenoid which opens and closes based on an electrical signal. The computer controls the length of open time. Since you mention "fuel rail" in your post you are likely talking Digifant. All of the Digifant II injectors fire at the same time.
I test the resistance of Digifant injectors with a multimeter. Each injector if disconnected and tested individually should test between 15 and 20 Ohms. You can test off the end of the fuel rail electrical connection. Disconnect the connector and test across the fuel rail end of the connector. Four injectors connected should test at 3.7 to 5 Ohms, three injectors connected should test at 5 to 6.7 Ohms, two injectors connected should test at 7.5 to 10 Ohms and you should get the above 15-20 Ohms with only one injector connected. I also test the harness for wiring faults. If the injector is out of specification replace it.
You can also use a 12V DC LED test light to test for injector pulse across the terminals of the wire end of the fuel rail connector. LEDs are polarized so if you do not initially get results reverse the connections. The test light should flicker when you have someone try to start the car. The flickering is fast and I generally cup the test light in my hands to see the flicker better. FR