when it's the door to (say) a toilet and half the point is that it doesn't have a window in

Frosted glass? Enough visibility to make out shapes, but not enough to see the stuff that's supposed to be hidden.

You could have an "Engaged/vacant" display[1].

[1] NB: one that works. It's REALLY ANNOYING when they don't.

The nearest gents' to my desk has two cubicles with engaged/vacant displays. One of them displays a little white blob for vacant and a red one for engaged (as you'd probably have guessed given that choice of colours). The other one is precisely the opposite way round, apparently because the door handle mechanism was put on back to front and nobody can be bothered to fix it...