Removing injectors is pretty straight forward, unless the 6600 has some odd set up.
First remove the return lines and the injector lines. Mark the order that they come off the pump, this will prevent a major headache when you put them back on. Then pull the injectors out. The easiest way is with a slide hammer.
As for testing them you'll have to find a diesel injection shop. The one locally will bench test injectors for just a few dollars each.