How can I indicate when a shower is in use?
This is the sort of thing that's trivial with new tech, and quite involved (expensive) without it.

It's easy with a few smart things. Assuming hot showers, a humidity sensor can trigger action. powering a dummy sensor with the exhaust fan circuit could trigger. or use a microphone sensor, or a PIR (motion detector) above the shower. Once the trigger's in the system, you can have any output you want, like a smart outlet or app notification. You can even setup "gates" around it like min/max trigger duration, debounce, etc, that would be god-awful to do in "analog".

The system can be a home-rolled node-red setup, or a commercial turn-key like SmartThings or HomeKit. The initial setup would be cheaper than fixing the problem, and then you could do lots of other neat things. If you don't have one already, it's the way to go, and if you do, then just add a new sensor and call it a day.