I saw a thing on TV some years go and LT track workers had a device with a load of bulbs in it all in parallel. It sat across the 2 live rails. When the current went off the bulbs went out. I think the assumption was that if all 6 bulbs went phut, it wasn't your day!.Indeed. This is a very pertinent consideration.
On the subject of track workers, I've heard of a device that can be held up to tell whether an overhead line is energised. I wonder whether there is something that can be devised for the third rail.
But a simple and effective device for detecting if the juice is off. I imagine a short circuiting bar would also be used.