You should be able to verify glow using a cheap Harbor Freight non-contact infrared thermometer. The exact temp is irrelevant you just need to verify that the glow plug is heating up at all. A couple of cycles of glow should heat the part of the plug exposed. Check the glow fuse if the car has one.

Once you verify glow you need to be certain fuel is getting to the nozzles, crack the lines at the injector and observe. If you've got fuel good, if not rig up an under hood fuel bottle with a couple hoses and eliminate the tank/lines as a potential problem. Still no fuel means filter or lift pump issue or other IP fuel circuit issue.

Another diagnostic would be using the block heater to warm the engine for easier starting. If it starts easier with block heat then that would point to a glow plug issue. With a single dead glow plug and a cold engine the OM 601/2/3 can be a bear to start.

What's the ambient air temps when this starting problem manifests itself? I've had an occurrence where over time water in the fuel gets trapped in the secondary(canister) filter's cotton medium and then when it gets cold enough that water saturated filter will severely restrict fuel to the IP.

Don't know how cold it gets where you are but synthetic oil makes a night -and-day difference with cranking speed in cold temps.