Best check in a manual. My Ford 1710 manual gives resistance specs for the individual plugs and for the wiring. Best to be clear on whether a spec like .9 ohm refers to the parallel plug connections or to each individual plug. A parallel connection of three .9 ohm loads draws about 40A. A .9 ohm load draws about 14A. Ahh Uhm--it's early and my arithmetic may be suspect.
Glow plugs themselves are either good or bad. They don't get weak, but contacts or bases can become corroded, which slows them down. A poor connection on one plug would be reflected in a resistance measurement that is higher than spec. If the spec is given for the parallel connection, the reading would still be higher than spec. However, then you would be left with finding which particular connection is poor. It's not good to have one doggy plug, because the other cylinders have to be over-heated before the doggy one gets hot enough to fire.
Another way of checking plugs is to remove them, clean everything. Then, connect them to the wiring, hold the base against the cylinder head, turn on the pre-heater and see if they glow. If it glows, the plug is good, but connections in the wiring harness still may be bad. ‘Holding the plug’ involves the usual precautions when working with electricity. You don’t want to weld a wedding band to your finger.