@Anthalas CO2 concentration at the fermentation trap outlet could work once you calibrate it, but a simpler method would be listening or watching for bubbles passing in a minute or less, hear them check again tomorrow or in 12 hours, hear none call ET, it's party time?
I used to open the cupboard and look, but that was in the days of black and white when caves were caves, but still a tried and tested technique if it doesn't work
