How can you check which glow plugs are not working!?

You don't need to remove the plugs just disconnect the bus bar between them and either use an ohm meter each should be about 3 ohms or just flash a bit of wire from the positive side of the battery to where the bus bar fits on the plug.If no spark then glow plug is open circuit

Yes, you can in a crude way. Remove the glow plug, connect a fairly heavy piece of wire from the positive pole of your battery to the place on the top of the glow plug where the normal wire goes, then hold the side of the plug where the thread is against the Negative pole of the battery.

USE A PAIR OF PLIERS OR SIMILAR TO HOLD IT!

The working end of the plug will almost immediately glow red if it is good. If it doesn't glow it is defunct.

Be careful because it gets VERY hot VERY quickly

you might as well replace them all as the other / others will be on the way out and they may be exchangable ( lower price if part is exchanged ) at a decent dealers try the motor factors before the main dealer . shop around and ask ! price you pay for running a diesel 50 /60 quid ?

voltmeter between supply cable and earth keep live connection away from engine block , get somebody to turn on ignition and test supply voltage . continuity test on plug

with resistance of < 1 ohm is good condition , any higher then it's knackered . careful how you go there are control units for the damned things . dont be taking em out and putting em back , in and leave em . is the dasboard light not coming on at all ?

I believe garages test them by measuring their electrical resistance. But if 2 glow plugs are kaput, then the other two won't be far behind. For peace of mind (and before the cold weather sets in) change all 4 of them.

stick your tongue on on end and turn engine over

easy to find which one isnt working