How to detect when the ignition on a diesel engine it turned on?

[sokoloff]

Alternately, if you're just looking to protect something from the starting transients, intercept the starting signal (disconnect from the starter solenoid and bring it to your device). Then, shutdown your device and re-create the starter signal out to the solenoid. You can do this with a timer or one-shot RC circuit if you need an orderly and lengthy shutdown, or if you just need a parallel signal, just tap off the starter solenoid (primary) wiring.

[CJay]

Easy, reverse bias diode across the fuel solenoid.

This WILL be your best and simplest 'engine ON' indicator by far and if you'd said it had an electric fuel solenoid we'd have all jumped on that one from the start. 

It'd be more unusual to find an old diesel that doesn't have a fuel cutoff solenoid though, so it'd be the first suggestion I'd have made.

They're there to stop runaway engines (even when the diesel from the tank is cut off it's not a guarantee the engine will stop) but aren't as necessary on newer fuel systems.

Shutting off the fuel or opening a 'dump' valve on the injection pump only helps if the runaway is due to a stuck injector rack.

2-stroke Detroit Diesels are notorious for running away, and continuing to runaway even with the fuel shut off, though it can happen with just about any poorly maintained turbo diesel, or even with a normally aspirated one with grossly excessive blowby and a high crankcase oil level.   Basically if oil can get into the intake and you don't have some means of totally shutting off the air supply, you risk an uncontrolled runaway with the engine consuming its own lube oil as fuel.

In the absence of an emergency air shutoff valve, you need something that will seal the air intake effectively.   If you are mildly paranoid and have ever been near a runaway diesel, you may choose to keep a pingpong bat clipped to the inside of the engine box lid.  Otherwise, grab a closed cell foam cushion or the Nautical Almanac and shove it up against the intake.   Don't try to use anything thin and flexible or the palm of your hand as the intake can pull a fairly high vacuum before the engine dies.

Mhmm, as I said, it's not a guarantee the engine will stop, I've seen a few that have runaway (and actually seen them do it) burning their own engine oil and only stopped when the engine ran out of oil and destroyed itself, as you say, the only sure way to stop it it to prevent it getting oxygen, I have seen CO2 suppression systems but never seen one used..

The voltage on the solenoid is a pretty sure sign the engine 'ignition' is at least switched on though which is what the OP wanted

[sokoloff]

Could be as simple as paralleling a relay off the starter solenoid primary and powering your PV monitoring system from a normally-closed terminal on the relay.

Starter engaged: relay triggers, powering down the PV system.
Starter released: relay opens, powering up the PV system (because it's on the NC terminals).

The relay can take the abuse of the starter solenoid (itself a relay) without other components generally.

[tautech]

if you'd said it had an electric fuel solenoid we'd have all jumped on that one from the start.

Sorry about that, I thought all diesel engines had cutoffs. How could you stop it without one?

Many/most mechanical diesel injector pumps (inline and rotary) had manual fuel shutoffs.
Trucks for example had a STOP bowden cable with a knob to pull to kill the engine. Tractors had either a throttle lever that went back past the idle position or a STOP rod linkage that connected to the fuel shutoff shaft on the injector pump. Key switch stops are comparatively recent (last 30-40 yrs) WRT the time diesel engines have been in existence.

I don't see the difference between "looking" at the terminal of the cutoff or "looking" at the start switch.

Correct, use the ON position at the switch but check the wiring with the thought of adding a diode if there's no ECU to manage the solenoid back EMF at the switch.