depends on how they fail, try giving it a sniff of easy start if starts then you know its fuel issue or a weak battery or starter, direct injection diesels will run no glow plugs should start if its all healthy, if it runs on a sniff of easy start you then know the engines is all good as such, i have had injectors play up where wont start on key but if use easy start will start and say running until switched off then need to repeat a sniff again to start,, leak off test on injectors will show up the bad one,
glow plugs can fail in a few ways, if positive to earth short circuit then would show high resistance but if go open circuit ie the electrode has burnt out a gap to break the circuit that will show as no resistance open circuit,, remove them stick across the battery with a jump lead see what glows and what dont bets way, or if car has north or 50k on it treat it to a new set as is one as failed the rest are not far off
IMPORTANT ALWAYS REMOVE GLOW PLUGS ON A VERY HOT ENGINE LOWERS THE REST OF SNAPPING OFF A LOT,
