Josh, to test the 12v parallel-style plugs, connect suitable sized wires from the battery to the glow plug terminal and its threaded part. Best to mount the plug in something like a vise first. Be careful not to have anything important or combustible near the tip. The tip should glow yellow-hot in about 5 sec. You can't test the series ones this way because they should get about 2v, not 12v.