On a tractor, (key and fuel off) I'd put it in road gear, grab the back tires and try to rock the whole thing back and forth. Doubtful you'd generate enough torque to hurt anything. Get someone to watch the front of the engine and see if it moves a little. Can watch the crankshaft or the fan.
Or, key off, fuel off, trans in neutral. Put a socket with a long breakover handle on the nut on the end of the crankshaft. Try to tighten it. Hopefully the whole engine turns.
If it were a gas engine, I'd say pull the plugs to let the compression off. On a diesel, you really don't want to pull the injectors except as a last resort.
If the starter will not turn the engine and you are pretty sure the engine is stuck, pull the starter off and see if it (the starter) is, in fact, turning when power is applied to it.
If you become 100% convinced the engine is stuck, there are things to try. But I'm for holding off until you are 100% sure there are no starter/electrical problems.