easy, fit an auxiliary switch to the pump via a contactor in an enclosure with 2 lights in the front of the enclosure, connect the supply to the green lamp which is pump on, and when the power drops to the red lamp this is done through a relay which has a normally open and normally closed connection which mean supply to pump off, easy peasy if your a spark
Well I suppose just a light across the pump would provide remote indication that the pump is running. However, that requires
EVERY user to understand what the light is telling them - and trust me, you will never "train" some users before the end of the universe.
In short, the light should come on in short bursts while the shower is running. If it doesn't, then the pump will fill up and overflow. But it needs the user to notice that it's not coming on - and that's where the system fails.
Ideally you'd fit a switch on the pump that indicates when it's not clearing water - not the same thing as not running. If it's a "submerged pump in a box" type like a Sanivite, then that's fairly easy as water level goes up when water flowing and pump not running, water level goes down when pump running, you just need a float switch to detect the water level too high.
From that switch I can see a number of options for action. One is simply to light a big red warning light and/or buzzer - that still requires user action to avoid overflow. Another option is to use it to turn off an inline solenoid valve and stop the shower - that stops the water flow but will "really annoy" whoever is half way through their shower, but arguably that's better than
REALLY annoying your downstairs neighbour (I've been that downstairs neighbour, or rather my tenants have, multiple times).
The alternative is that you get into timers and flow switches. When the shower has flow, reset the timer whenever the waste is flowing (ideally via a flow switch as you can have the power on but pump not running*), if the timer times out, alarm comes on and/or shower gets shut off via solenoid valves. Set the timer too long and it's slow to detect pump failure, set it too short and you'll get false alarms.
* Sanivite/Saniflow pumps are prone to capacitor failure - then they just hum. Also more prone to switch failure. I've repaired a few with one or both faults.
Our other property has a Phlexiflow pump (earlier version of
this) fitted for the previous owner as part of a low-entry disabled shower conversion. That's a direct pump that sucks water (uphill) from the shower waste, and if it stops working then the water just backs up into the tray - obvious to the user. It has no level switch, it relies on a flow switch in the shower supply, and a controller that you configure so that it will clear the maximum flow in bursts, and otherwise gurgles a bit as it sucks air.
