It's a DC voltage, but it's a pulsating DC voltage, so don't expect to see 12V.

A "noid light" is pretty inexpensive. Disconnect the injector, plug the noid into the harness, and its light should flash. Here's a complete set. They're also available individually from Snap-on, Matco, Mac, etc.

But that'll only tell you whether the injectors are getting their firing voltage. It won't tell you if that's what's causing the clicking. To do that, you have to listen. An automotive stethoscope is also pretty inexpensive and is a real help in isolating noises. An alternative is to use a big screwdriver. Put the handle against your ear and put the tip on the valve cover or injector stem. You'll hear exactly where the sound is coming from. Just be careful not to put the tip on anything that can shock you (electrically) or near any moving belts, etc.

FWIW, I'd put my money on the valves as being the source of the ticking sound.