If you want to start doing your own troubleshooting you need to invest a few $ in a couple of tools.
First would be a 12 volt test light for less than $20 on line or at any auto parts place.
The first test you would do it to connect the alligator clip to the battery negative post.
With the key in preheat position, touch the test light tip to the end of the glow plug where the wire connects. Glow plug remains installed!
The test light should light proving power is reaching the glow plugs
Next, disconnect the wires from the glow plugs. Clip the alligator clip to the disconnected wires and touch the test light tip to the glow plug electrical terminal. You should see the test light bulb light up perhaps not full brightness. Do this for each glow plug with key switch in pre-heat position,.
If the test light reacts, you have established the glow plug is likely OK. Repeat for each glow plug.
If one or more Glow plugs do not react the same to the second test, one or more GP's is faulty
Dave