a constant +12v. This assumes your car has NOT been mod'd to turn off the +12v when the ignition is off. If not sure run the voltage test both ways (ignition off and then on but car not started). There must be +12v feeding the injector for it to work right.
A simple LED (tester light) can be used for this too if no voltmeter is handy but I wasn't sure on your level of electrical know-how?