You can use a mechanics stethoscope. It has a straight bar instead of a bell for listening. Or use a long screwdriver placing the handle part on your ear. Place the tip of the bar on the injector while the engine is running. You will hopefully hear a click each time the injector opens. This tells you that they are at least trying to work. Then you can disconnect the electric connector of each injector. One at a time. The engine should run ruffer with one disconnected. Go down the line, one at a time. If you find one that does not seem to make it run worse. That one injector maybe plugged or not functioning correctly. That's at least is a start. Something that doesn't involve major wrenching. Good luck.