You can inspect your orignal fuse by removing it. Any defect will be obvious as it is only a calibrated strip of metal. Saves hunting for a fuse if not required.
In my opinion though your problem is more likely to be an old tired glow plug relay. Seek out a hulk at a wreckers for a used one or try one of the used parts from our used parts forum. Those relays get tired with time. Just my experience and you would need to substitute the relay anyways before troubleshooting further if the fuse strip is intact.
The above said at some point of developement of the glow plug circuit on 123s the relay light would not come on if certain glow plugs had failed. So quickly check all your glow plugs for continuity. I do not know what year that started in. Not coming on would be a constant in my opinion. Since yours again is intermittent beg, Borrow, or aquire a cheap relay for a test.