even though the spec is 9~ 14 PSI, most TBIs won't run very good if the pressure is at or below about 10 PSI. anything below 9 and they barely move.
but i have seen 1 that ran ok at 6 PSI. it did have a complaint of lack of power, above 70 MPH.
they make an adapter that replaces to fuel filter to check the pressure, i believe Snap-on sells it.
or ATD makes a pretty complete fuel pressure kit.
i have one and so far i haven't ran across a car i haven't been able to hook up to.
it has the adapters needed to hookup to the pressure line at the throttle body.