Fuel Pressure Gauge
A Sears/Craftsman gauge costs $35-55, but many parts stores will loan/rent tools like this for a deposit.
This is a '98 4.6L car, but the idea is the same for all EFI vehicles. Some just don't have a test fitting, which makes connecting the guage more challenging.
For '90-96 F-series & Bronco, read this:
For '84-89 EFI trucks, see these captions: