I could be wrong, but there are a couple of guys in the UK who are the boffins on these. From memory DMC and one other (sorry, can't remember) but DMC always chimes in with diesel advice and he knows his stuff. AND knows electrics back to front as well.
I have had little to do with diesels (have a diesel boat, and had to fix a blokes Toyota diesel timing belt........... AND HE WAS SUPPOSEDLY A DIESEL FITTER?????) but I know electrics.
DMC will confirm, but the glow plugs draw a lot of current, so just beware testing them, because they may be very low resistance, and resistance that low is hard for the average meter to measure.
I have half a dozen digital meters, and the most reliable I have for low resistances is either my analogue moving coil meter, or a dedicated LCR meter. The best for low resistances is a megger (bridge megger) that will read down to fractions of an ohm. I had those at work, but no one has them "kicking around at home."
When I was measuring field coils for my motor bike generator, they all measured exactly the same resistance with every meter, about 1.5 ohms. BUT they measured different inductance, so an inductance meter will tell you if any coil has a partial short, ie "only half the coils working". A dead short easy to find, but a partial one is harder, and that is what the bike fields had; 2 out of three had internal shorts/only half the coils were in cct.
That is assuming the glow plugs are "coil configuration". DMC will know.
So, the point is, unless they have a "short to case", they will likely all measure a very low resistance normally, and hard to tell any slight variation.
I suppose if you have a reliable current meter, you could test each one and see what current they draw, and if all the same/within spec should be ok. But (again) many won't have a current meter that can measure that relatively high current. Plus, any heater draws far more when cold, then decrease as they heat.
This is likely suck eggs, but if you do decide to check the current, remember that current is measured in series, and in that config the meter has NO internal resistance, so if connected ACROSS 12 volts, it will be a short/IMMEDIATELY blow up the meter. If that is obvious/suck eggs, then sorry. But I would rather explain what is obvious to some than to have you blow up your meter/damage something else, then say "I wish you'd said".
Measuring very low resistances (a 12 volt heater drawing 12 amps has 1 ohm resistance) is often beyond the average meter, as they can't see the difference between a short at zero ohms and a working heater/bank of heaters at 1 ohm.
I defer to DMC for specifics, he knows far more than me about actual layout/equipment on diesels.
