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A fuel pressure test will help a lot. Both running and standing fuel pressure readings would be important. You would especially want to look at the pressure AFTER engine shut-down to see how quickly it drops. Theoretically, it should maintain pressure indefinitely. If pressure drops off within a minute, the pressure regulator and/or injectors are suspect.
If the pump does not operate for the two-second prime cycle when the ignition is first turned on, the pump relay may be a problem.
If the pump runs the prime cycle and starting is made easier by opening the throttle 75% or more, the regulator or an injector may be leaking and pooling fuel in the intake.