plugs for m1008

i know that its been talked about before but im a little slow on understanding it. but how can you test the glow plugs to tell if they are in the correct ohm range and functioning properly?

you can pull them and put them in a vise and put power to them. Only the tip should glow. You can use a test light without pulling them but I forget how and it will only find dead ones not semi dead. I went through this today. Some idiot put 24v glows in my CUCV. No wonder it was hard to start. It starts like a SOB now. You would think the different connectors would have tipped them off but they actually made adapters to use the wrong glows....duhhhh

i have been told to check the ohms and they should be between .5 and 3 ohms. i just haven't figured out how they are getting that reading. when i test mine with the positive lead on the plug and the ground to the engine i get an open curcit, not .5 to 3. any help?