I have a 2cyl z751 diesel. I made an electrical print. I measure 1.0 ohms from glow indicator to ground. That goes thru 2 plugs in parallel, so I induce that each plug is about 2.0 ohms.
You don't need a great VOM, just a cheapo HbFreight $3 digitalVOM. On the lowest scale, short the leads together, firmly. Should get about 0.6 ohm, and whatever that measure is like a tare weight. More difficult is getting the probe on a known good ground (battery neg easiest), and on a glow plug terminal. Mine are painted, so scrape off some paint. Subtract the "tare" (0.6ohms?) from the measure, and multiply the remainder by no. of plugs. If all plugs are good, that is the answer per plug. If you measurement is not reasonable, then you will have to electrically disconnect them from eachother, and measure each one.
The OEM print was for a z751a in the L185. I have a z751-HY (gray) on a L1511. The OEM print showed the plugs in series, and I can't remember if I recalculated after I found that they are actually wired in parallel. I did measure 1.0 ohm, so each plug must be 2.0 ohms, and my attached print needs to be corrected. I think the OEM print has got to be wrong. It would be difficult to wire plugs in series, and I see no advantage of doing so, unless engineering wanted to prevent one starting with one plug electrically open.