...
If you would be qualified to work with such dangerous voltages, you should not need to ask help on this forum....
...
Bullshit and gatekeeping. There could be a whole host of legitimate reasons that he can't / doesn't want to ask a fellow (local to him) engineer about this. Time constraints. Weekend. He's a solo operator at a small company. Just moved. In any case, asking a fellow engineer is
precisely what he's doing here right now!
The way health and safety advocates overwhelm discussion on this forum sometimes is very annoying. We all know there are risks. We know that the risks increase with voltage and current.
If you have a safety concern
and an answer to the question, then voice your concern, answer the question, and move on.
If you have a safety concern
and no answer to the question, then don't bother.
As for the original question, my first attempt would be a pair of hall effect sensors. Or, because presumably the current through the contacts is so much higher than the current through the solenoid that it would saturate the hall sensor on that side, use a current shunt instead on that side. So when there is no current through the shunt (through the solenoid), but the hall effect sensor is still high, then you have welded contact situation.