Go to Harbor Freight and get the about $6 Digital Volt/Ohm Meter (Multi Meter).
The Meter is best but if you have a 12 Volt Test Light you can use that.

I am assuming that the new Glow Plug Relays look similar to mine. There is a Flat Copper Wire and the Strip Fuse that should have Battery Voltage all of the time.
There was some Glow Plug Relays that had a Circuit Breaker built into it instead of a Strip Fuse.
Screws on the Strip Fuse have been know to be loose and the Strip Fuse itself can develop a Hair Line Crack.

You can use the Meter or test Lamp to determine if you are getting Voltage up to the Glow Plugs.

If you go to the Diesel Giant Website there is a Pictorial on a quick test using the resistance Function on the Volt/Ohm Meter listed under Glow Plug Repair. It looks like the Page is not opening up but you need to scroll down the page.

If you are not gettin Voltage to the Glow Plugs you will need to remove the Electrical Connector from the Glow Plug Relay and see if when you turn the Ignition Switch to Preglow that you are getting Voltag.
Unforunately I don't know what you Connector looks like, what color of Wire, or which number of hole on the Connector comes from the Ignition Switch.

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84 300D, 82 Volvo 244Gl Diesel