The check valve is part of the fuel pump assembly located in the gas tank. You might have a leaking fuel pressure regulator however and that's on the underside of the fuel rail with a vacuum line going to it. Sometimes those leak gas into the vacuum line, will see it if you run the engine, shut it off and pull off the vacuum line. Before going further, it'd be best to put a fuel pressure guage on the fuel rail fitting and see if it is losing pressure. It will eventually go down but should hold pressure when you shut the engine off, not quickly drop to zero.