You could measure for voltage there during the glow plug cycle. It should be between 10 and 12.6 volt and if working properly and drawing a lot of current like they should it would be closer to the 10 volt than the 12.6. If you have voltage, you can remove the rail, and isolate the plugs and run a resistance check on each plug. Mine on the Kioti are supposed to be .43 ohms when cold. that is less than 1/2 of an ohm.. so I would think yours would be somewhere in that ballpark also. If they measure open, then replace them. If they are a dead short replace them. You will need a good digital ohm meter and understand how to read it, and start by shorting your ohmmeter leads together and see what they read, and figure that into your calculations when you read the glow plug center pin (that was hooked to the rail) to ground. Of course if you have no voltage on the glow plug rail during the glow plug cycle, then look further up stream to the glow plug relay or the glow plug timer. I am not familiar with your machine to give you specifics but they all work similarly.
James K0UA