When the injector gets 'juiced', it makes a pulsing noise. You can put the end of a long screwdriver, socket extension, wooden dowel, etc. up to the injector and put your ear on the other end. If it's receiving voltage, and the injector is operating, you'll be able to hear it. Or if you have the 'noid' light tool as raz54 mentioned, you'll know for sure if it's getting power!