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you should be able to take a screwdriver like an stethoscope up to each of your injectors and here the vibrations from the opening and closing. You should be able to tell if the injector is opening or not from this.
As for finding and injector if you're just looking for a stocker try asking on the Buick boards I'm sure there are lots of guys on there with extras laying around where as TTA guys tend to hold onto all of their stock stuff.