P.Haiges. All it takes is a cheap digital meter that has a millivolt range on it. Read from the glow plug harness connections on the harness plug once it is unplugged from the glow plug relay if you have one. Makes that part very simple.
If you do not have a plug in your glow plug system then you would read each plug by disconnection of the wire as you suggested. The only caution is to use the cylinder head as the ground for the meter for either approach.
Using the battery negative terminal or a chassis point for the ground will let things like the alternator mess up your voltage. The wiring resistance lets the interference develop. In fact it makes the digital meter hunt. Done properly the meter is rock stable.
Using an oscilloscope would give you no advantage. This is low tech and requires your active participation if readings are suspicious..
The primary variable in this system is to designate one plug as a calibration plug to verify the abnormal readings are being caused by something else other than the glow plugs themselves before passing that point. This is only if you have a problem and are checking with accuracy.
Now we are getting fussy though. If a cylinder has a fairly bad problem or whatever the reading will be way off as there is a major difference in voltage.(heat output from that cylinder).
Some glow plugs will generate different voltages while exposed to the same heat constant unfortunatly. So it makes for a little more work to use one plug as the calibrated refference.
All this is really doing is giving us an additional tool as used on larger diesel engines. The ability to indicate the running temperature in each cylinder compared to the others. Pyrometer?
We have had a few gentleman deal with serious knocking problems succesfully with this approach. Perhaps some of them could repost if they read this. Their injection pumps got tweeked. After the injectors were swapped I hope. Problems were gone in the majority of cases.
Even just used as a quick injector test I believe it has validity. Since it is a dynamic test as you mentioned it should be superior to any static test for quite a few things.
It is best to post any indications of a problem indicated by this method on site before dealing with it. That way nothing is overlooked. On engines with fairly recent glow plugs,smooth idles, good fuel milage, and Good performance. All the glow plugs indicated the same voltage within .2 millivolts.
The average readings from older mercedes engines will vary from 7-14millivolts depending it seems on who made the glow plugs. This in itself is not important as we are using the glow plugs for comparison purposes.
Again sorry to be a little long winded. New people who do not have your experience may read this and I want them to have a grasp of the milli volt approach as well.
If more and more people post their readings or try it this will become more mainstream I believe. . People like Sam and Yellit have read their engines I believe. Quite a few others come to mind as well.