This seems so basic that I may just be missing something obvious but...
how the hell can you find out if spellcasting is toggled ON or OFF for your henchman?

The basic thing your missing, is which game.
If it's NWN2
You highlight the character, click on the eye thing at the bottom left of screen & choose Character.
Then click the behavior tab and scroll down.
See pic.

If it's NWN1, I don't think you can in an unmodded game.
From what I remember Sharwyn, casts all her spells at the first stray dog she sees, and has nothing left for the main battles.
But it's been a while since I played, so I might be wrong.