Fortunately, milk kefir is a very stable ferment, which rarely goes bad and can protect itself well against harmful microorganisms.
If your milk kefir does go bad, you will definitely notice it with your senses. Your instincts will protect you.
Mould, for example, is definitely a reason to discard the kefir. You can see it on the surface as a furry coating in white, green, black or other colours. Kefir actually only goes mouldy if you don't feed it for a long time and the cultures die or the culture is weakened by other means. It is important to note that in the case of mould, unfortunately the entire culture must go to the compost, as mould can produce very harmful toxins.
If your kefir suddenly smells unusual, this can also be a reason to dispose of this batch. However, a slightly cheesy smell is not problematic.
In general, it is important not to use milk that is already bad. It will no longer make healthy milk kefir. Milk with too few tubers (less than a 1/50 ratio of tubers to milk) can also become mouldy, as the microorganisms need longer to colonise everything. In this case, it can help to stir in some ready-made kefir as a starter in addition to the tubers.