if you have a pair of needle nose pliers and an old but good wire you can test damn near everything.
Take the wire straight from battery positive to the plug on the injection pump, Use the pliers to jumper the relay for glowplugs for about 8 Mississippis', and then use the pliers to actuate the starter relay. Should crank over and start.
You can take a test light to each glowplug head and if it don't light then 100% that plug is dead. These trucks can start with about 5-6 working minimum, in OK weather, if its close to freezing forget about it when under 6 working plugs unless your truck and batteries and starter are top notch.
Now if it doesn't start:
Doesn't crank: bad relay or starter, (hopefully the teeth on the flywheel are OK - I can't recall a case of that happening but I'm sure it has), or the cables are bad/dirty. Should hear something - relay noises, starter noises. If you hear something spinning/whirring, the starter is bad - the thing that makes it engage to the flywheel isn't working probably.
Testing relays side note: If you are ballsy you can bypass the relays with a big piece of metal - I've had to do this before. If it starts to crank the relay is shot or making poor contact (corrosion). This can tell you if the relay for starter/plugs is bad, and get you out of a jam if either fail when you're out and about especially with the factory glow plug controller that ***** the bed when 1 plug might be dead or corroded.
Cranks but wont start: keep cranking, see if fuel vapor is coming from the exhaust. Should be grayish white. No smoke means no fuel, either the fuel pickup in the tank is bad (so it isn't picking any fuel up - happens all the time on these old trucks), the fuel line is clogged (with rust, algae, or prior-mentioned fuel pick up), clogged fuel filter, or IP shutoff is stuck shut. Test the shrader valve on the filter head, see if fuel is gushing out when cranking. That'll help you find out if its before or after in this case. You can also try tapping on the IP gently to see if the shutoff was stuck shut. Should get a spark when giving power from a wire.