There is usually a capped schrader type test port on the injector rail just for this purpose. Unscrew the cap and the test valve looks just like a tire valve stem. There are gauges with about three feet of hose that will screw onto the test port. Run the hose out the back of the hood and tape the gauge to the windshield so that you can monitor it from the drivers seat. Make sure the hose isn't in direct contact with anything real hot like an exhaust manifold. Start the engine and make sure there are no leaks before closing the hood and taking off down the road. There should be enough "squish" in the hood seal that it doesn't crush the hose.

Your local auto parts store may have a gauge set that you can check out for just a security deposit.