Depending on which FASS system you have there is a pressure port built into the pump.
150 FASS System - Just above the fuel filter, attached to the filter base is a red or silver block or cube, It may also be a silver, hockey puck looking piece. On the back side of that is a 1/8" brass, allen head, plug marked with a "G" or a "P". you can port a gauge to that.
95 Series - Just above the fuel filter there is a 90 degree brass elbow fitting that feeds pressure to the engine. On the back side of that is a 1/8" brass, allen head, plug marked with a "G" or a "P". you can port a gauge to that.
HPFP - There is a 1/8" brass, allen head, plug marked with a "G" or a "P". you can port a gauge to that.
Hope that this helps.