You'll need a fuel pressure gauge. Many auto parts stores borrow or rent them. Many engines have a test port on the fuel rail on top of the engine that looks like a tire valve. For engines that don't have a test port, you'll need to remove the rubber fuel supply hose where it comes from the body to the fuel rail and fashion a tee to attach the gauge. Most better gauge kits come with the parts to do that.
The actual pressure can vary too according to changes in engine vacuum. Most tests specify the amount of pressure you should have with the engine not running. You'll likely have to bypass or activate the fuel pump relay to keep the pump running when the engine is not rotating.