The absolute BEST way to test the glow plug relay itself is to do a voltage drop test across the high current leads. This will tell you whether or not your relay is in good shape, marginal shape, or "I'm surprised the truck even starts!" shape. Make sure the engine hasn't been run for more than 8 hours. This will give you plenty of time to see what the test results are because the PCM will keep the glow plug circuit active for more than 30 seconds.
Using a volt meter (set to DC volts and not ohms or amps!!!) with alligator clips on both leads, attach the red one to the side of the relay with the big black/orange tracer wire and attach the black lead to the other big post with the brown wires.
Turn the meter "on". It should read battery voltage. If it doesn't, you've either got an open circuit in the power feed (big black wire) or you have 8 burned-out glow plugs, an open in the wiring, or the fusible links in the glow plug wiring that are burned up. Once you've established that you have battery voltage reading on your meter at this point, go ahead and turn the ignition switch to "run". Don't start the truck though.
Run around to where you can view the meter display. What does it say? In an ideal world, the meter should read 0 volts. That means that there is no resistance (voltage drop) between the two contacts and the contact plate in the relay. Rarely is this the case - especially on an old relay. If you are dropping more than 1.5-2v across the relay, it is bad and needs to be replaced because you are losing that much voltage across the contacts and your glow plugs are not getting as hot as they should be. Heck, any more than .5-.8V is excessive......
If the meter still reads battery voltage, you need to find out whether or not the relay is even being commanded "on". Again, use your meter to check for voltage. This time, go across the two smaller terminals on the relay (control side). Turn on the ignition. Do you see voltage? You should. Is the relay clicking? If you see voltage on the control side and the relay does not audibly "click", then you need to check the resistance of the solenoid coil. Do this by disconnecting one of the smaller terminal leads and using your meter set to ohms ( the symbol looks like a horseshoe). You want to see about 5-10 ohms. If you see anything more than that, then there is too much resistance and a strong enough magnetic field may not be created to pull-in the contact plate. If your meter displays "OPEN" or "OL" or anything pertaining to an open circuit (an open circuit IS NOT 0 ohms...that is a short and is also a problem) then you have found your problem and the relay is junk. However, any value out of the range of 5-10 ohms, especially an open or short, means that the relay isn't happy.
If the relay clicks, but the meter still displays battery voltage across the big terminals when doing a voltage drop test, you need a new relay because there is no current flow through the relay when it's commanded on. Disassembling the relay will likely reveal to you that the contacts and the contact plate are burned very badly.
Hope this helps!