There is a schrader valve on the fuel rack above the injectors where you can connect your test gauge. I don't have my manual in front of me but if I remember right it should read a constant 34psi (or there abouts) and should not leak down too quickly after you shut down the engine.
Finally got around to looking for this schrader valve. Doesn't appear to be one on my '93 3.9. Shop manual says to attach a guage to the fuel line at the fuel filter. Anyone know what kind of guage will fit?