Because 1.3u does not have a built-in fuse, one simple way to add a fuse is to connect it in series on the COM wire. So COM terminal -> fuse -> COM wire.
Regarding ‘an indicator that a station is not working’ — a shorted solenoid can be easily detected electrically (or at least manually by measuring the coil resistance). But there might be mechanical issues like solenoid getting stuck etc. Ultimately if there is a soil moisture sensor, that can tell you if the plant is getting the water it needed.