I'd be real careful about that if employees are going to be using it. Someone gets hurt and OSHA will have a field day with it.
I like the lamp suggestion better, but that does not tell you the voltage.
True. But it tells you if it is live.
The image I posted with the wee indicators is LED cluster lamps. Of course they can fail though I have no experience of any doing so. With a 30,000 life and just using them for test purposes failure seems remote in a lifetime.
But you can't rule out the possibility. So stick a couple in parallel. They are tolerant of wide voltage variations so the probability of both failing at the same instant without giving any indication is vanishingly remote.
And, of course, you'd check if they were operational immediately prior to depending on them for the test.