If you know where the injectors are on the engine, you can find the glow plugs. They are pretty much the only thing with an electrical wire going to them on the side of the engine block, and are a few inches directly below the injectors.
The wire has a rubber boot on it that goes over the glow plug. You remove the wire/rubber boot from the plug, and put one meter probe on the end tip of the glow plug, and the other to a good engine ground (any exposed metal on the block will do)
They should be somewhere around 1.5 ohms. Pretty much anywhere between 1 and 5 ohms is still okay. When failed, they are usually no reading at all, or up in the thousands to millions of ohms. Occasionally they read a dead zero, but that seems less common.
Depending on your alternator, the driver's side can be reached from on top of the engine, reaching down behind the alt. Can't do that with the 400 amp, but 200 amp it's easy.
Passenger side can access them by removing the doghouse cover and reaching into the space. Might be a little tight, but doable.
I was only able to reach the rear 3 plugs on either side like this, but unless you're talking sub zero weather, 6 glow plugs will start the engine just fine and with minimal smoke. Mine was starting at freezing on only one confirmed good plug, 5 bad, and 2 unknown condition.
EDIT: with the multimeter do make sure to touch the leads together beforehand to find wire/probe resistance to subtract from your reading on the glowplug. I have seen some meters that have more than 1 ohm resistance through the wires/probes and that will throw off the measurement enough to matter sometimes.
