How do I determine source of fuel pressure problem?

Gas or Diesel? Low fuel pressure can be a restricted fuel filter or a failing pump

It's gas. 5.4 V8. I just had a new crate motor iinstalled. Haven't driven 300 miles on it. I'm hoping not pump but I can't hear it engage with ignition and I can get it to crank with starting fluid directly into intake.

Just because you hear pump,doe,s not mean it is running,check the fuel pressure at fuel rail,

I did not hear the pump engage. I don't have a tool to test pressure at rail but I did depress the check valve and did not get any fuel at all.

If you are not getting fuel.first check the fuel relay,then fuel filter ,then the pump.

You can rent a loan a tool at part store,fuel pressure checker,it hooks, in the the fuel rail,but if you check it just by pressing stem and did not get gas,i think your fuel pump may be bad.

The relay seems to be ok. I'm gonna replace the filter in the morning cause it's due anyway. I just had a new motor installed and I'm starting to wonder if I may have a bad ground somewhere