I’ve made some homemade yoghurt following the recipe in the book but am wondering how I can find out if it contains the good bacterias???

I’ve just been reading on the Easiyo website that after a few days, the bacteria in yoghurt reduces considerably. Can anyone say yes or no to this? A batch of yoghurt normally lasts me a week therefore, this means that by the end of the week the effectiveness of my yoghurt is reduced.