Its not really a proven way of testing them - just for listening to them. My old Pug used to make a lovely rattle when it was running, and I always assumed tappets, but it was the injectors. Popping the flat end of my monster screwdriver and putting my ear to the other, you could hear the injector firing on each stroke.
Get a fire extinguisher...The best way to test injectors is to remove them - but keep it attached to the fuel rail, put 4 jars under each injector (for the love of god, disconnect the spark plugs), and get someone to crank the engine. Watch the spray pattern, intensity, volume (its pretty obvious to compare). This is pretty risky - fire and all that, but unless you have a spare K-Jet system lying around to test them on, its the only way.
http://www.dailymotion.com/video/x67m_kjetronic-video
