Diesels are simple, at least in concept you give them fuel, air (which should free flow on non turbocharged engines) and compression and they run, they may run poorly, but they at least try to run. Personally I tend to check the return line first and work my way back from there by group, 90 degree fittings are particularly prone to clogging.
1, check the return fuel line wherever it is easiest to get to after the injectors (a clogged return line closer to the tank will usually cause poor running due to back pressure)
2 check the see if your getting fuel to the injector /bleed the injector make sure to wrap a rag around the fitting and remember if everything is working right this fuel will be at very high pressures (high enough to inject it through your skin and cause you to have a real bad day)
3 Check the inlet side of the injector pump, this should have low pressure fuel from your lift pump
If still nothing work your way back through the filters, pump, pick up. etc.
Sometimes it is necessary to start diesels engines that have sat up for a long time on gasoline fumes, this can be done by soaking a shop rag/paper towel in gasoline and holding it near the air intake. This is dangerous, if you do it make sure the grid heaters and the glow plugs do not get turned on. When I have been forced to do this I like to wash my hands good first to make sure nothing flamable is on them, and hold the gasoline soaked rag with a long pair of pliers (note it only takes a little gasoline, maybe a tablespoon full, and it evaporates very fast so you have to hurry)
Ike
p.s. don't let the rag get sucked down the intake when the engine roars to life (vise grips might be good here)