QWORK Universal Fuel Pressure Test Kit, Dual Scale Gauge (0-7 bar/0-100 psi) with Multiple Adapter Sizes, Brass Connectors

You can get fuel pressure gauges that go between the pump and the rails. You just disconnect the hose from the rail and put the tester in line. Like this one https://www.amazon.com/QWORK-Universal-Pressure-Multiple-Connectors/dp/B0DRVKYZ15