You can determine if the bacteria in your yogurt or kefir are alive and active by putting the yogurt or kefir into a clean glass, cover the glass tightly with plastic wrap and leave the glass overnight in a warm place such as near a warm heater, but not too hot. Come morning, if the wrap remains flat, it means the fermentation didn’t start and the gases didn’t form because the bacteria were not alive or active.
Not too sure that would work Able.
The fermentation by bifidobacterium and lactobacillus ought to be anaerobic which produces lactic acid only. No CO2 released from bacterial anaerobic respiration of the strains concerned here.
I think a better test would be to take some of the yoghurt and mix it into a larger volume of milk and then see if it will turn that milk into yoghurt. (under correct temperature)