I mentioned Archoil AR 9100 in an earlier post.
There are lots of threads on fabricating a fuel pressure gauge. Not hard to do. Look them up. The only other option is to take it in - a dealership has the tools to check fuel pressure.
Post videos on YouTube and then post the link in the forum.
I feel like your answer is somewhere in the possibilities that I have already mentioned and you are just going to have to roll up your sleeves and eliminate possibilities one-by-one.