I was able to narrow a glow plug problem down to the specific component using some of the following steps. I was not sure if the problem was the relay or glow plug(s).

1) Remove the cover on the glow plug relay connectors.

2) Visually examine the fuse strip. In my case I actually removed it for inspection - probably not necessary.

3) Verify battery voltage at the incoming heavy battery line to the glow plug relay using a multimeter. This happens to be one side of the fuse strip. Also verify battery voltage at the terminal on the other side of the fuse strip (helps check electrical continuity of fuse strip).

4) Pull the larger plug connector from the glow plug relay. The wires in this connector go individually to all your glow plugs (talking about the newer pin/parallel systems).

5) Turn the key switch over to ON as if you were warming the glow plugs. You should now have battery voltage at each terminal on the glow plug relay that you have exposed by removing the connector in step 4). Voltage should be removed and you should hear a click after the glow plug relay times out.

6) Inside the connector, each wire going to the individual glow plugs should show a low ohm connection. (set meter to read resistance, of course).

7) Here's the harder part, because it requires a DC ammeter that can handle some current. I happen to have one that can handle 20 Amps. (If you don't have this capability, I'll describe a seat-of-the-pants alternative approach.) Measure the current that each glow plug draws by connecting the DC ammeter between the battery supply line on the glow plug relay and the individual wire connection inside the removed plug. Each glow plug should settle down into a current draw somewhere around 10 amps.
[You might also want to run this test using the glow plug relay and the appropriate pin the glow plug connects to - in order to insure the glow plug relay is able to handle the current load.]

8) A less precise alternative to 7) above is to use a piece of wire and visually gauge the spark drawn when connecting the glow plug. [You have to be careful doing this, and must absolutely know what you are connecting - you don't want to mistakenly ground the battery supply through a small wire - the glow plug has to be in the circuit.]

I had one glow plug that drew considerably less current than all the rest. It also seemed to heat only halfway up the length of the plug - not at the tip of the plug. It drew less than 2 amps and visually produced a much fainter spark when connecting to the battery supply.

I suppose if you don't have a multimeter, you could use a 12-volt bulb to test the glow plug relay output voltage.

Ken300D