I would definitely test them before replacing them. Even if you just test for power going to them with a test light to make sure power is getting there. From there you could ohm them 1 at a time between the stud and ground and check for open circuitI just had mine off to test them...they were "NGK Y106V". The manual which is linked on the forum here shows a testing procedure to do before replacing them.
If spark plugs are any guide, NGK makes the best IMO. I personally would only use NGK spark plugs in my vehicles, and definitely the same for the glow plugs in my new-to-me 443 with a Kubota D750.I would definitely test them before replacing them. Even if you just test for power going to them with a test light to make sure power is getting there. From there you could ohm them 1 at a time between the stud and ground and check for open circuit