how do you test rail pressure on injectors - truck won't start

Connect appropriate fuel pressure gauge to test site usually located on injector rail under black plastic cap that prevents dust and foreign debris from depressing Schrader valve accidentally spraying raw fuel onto the engine. Check for manufacturer's specs for pressure reading, usually around 55-65 psi. Also check for blown fuel-pump fuse/ PCM / injector/ fuses. ASD relay failure can also cause fuel pump not to run! If everything checks out on many higher mileage vehicles, that have been ran low on fuel, pump has overheated and seized. While someone is cranking engine over install noide light into injector harness connector , it should flash. Injector noides can be purchased at harbor freight.