Has anyone any advice please.
Making a sensor is not generally a do-it-yourself project.
1. The level of the coolant in a diesel engine radiator.
There should be enough to cause the overflow tank level to move up and down. Much easier to put a float type sensor there.
2. The temperature of the coolant.
Buy a commercially available sensor.
3. If the cap is screwed on correctly. (i.e it's on)
No way to detect crossthreaded installation that I know of. If all you want to know is whether it there, or not, a simple switch that is pressed when the cap is on should be sufficient.
I do not know how toxic or caustic it is
Spill some, and see what happens to animals that drink it. You'll know.
There are sensors that can stand up to the mixture, though.