Welcome to TBN. Individual glow plugs are tested by removing the bussbar strip connecting the plugs. With an ohmeter, test each one between the plug's post and the plug base. 1 ohm of resistance should be present on a good plug. This takes into account that you have 12 volts coming to the glow plugs when they are activated. To remove a plug, unscrew from head. Put some penetrating oil on it prior to doing so.