Two ways to test diesel glow plugs

Take the glow plugs out of the engine block and measure their resistance. Should be 0.8Ω to 1Ω (don't forget the resistance of your meter leads, so 1.1Ω or 1.2Ω is okay.

To double check, do the glow test. I smeared a bit of grease on the end so you can see it smoke, just in case it doesn't get hot enough to glow.
Once it glows red, disconnect the power, you don't want to burn it out. I used the -ve terminal of the battery to put the jump lead on, just in case you drop it, you're not going to short the battery out.

You can also do this test if you have flexible cables going to the end of your glow plugs. Put the earth jump lead around the glow plug, and put the ignition in to position [2]. The good plugs will glow. This will also tell you the circuit to the glow plug is working OK.

Видео Two ways to test diesel glow plugs канала Al Mann