The fuse needless to say with an ohmeter. Or with the ignition on 12 volts on each side of it. If it is open check for a plug with a dead short. You can read the individual plugs through the harness connections. Good plugs will read perhaps an ohm or so a dead shorted plug will read zero resistance pretty well. No shorted plugs? The fuse could have died from just old age. Thats a start. During the check of the plugs keep an eye open for any glow plugs that read infinity or high resistance. They are shot.