There is a special service tool available from OTC. It uses a banjo fitting and a longer banjo bolt so you can attach it on the end of the fuel rail.
Or you can simply cut the high pressure hose and T a fitting into it for the pressure gauge. When you are done, you just patch the cut with small piece of pipe and hose clamps. I had one set up for this and loaned it to a guy in FL about a year ago and he refuses to send it back.
The fuel pressure should be 43.5 all the time on these engines in stock configuration.